485BPOS

Registration No. 333-133173
File No. 811-21888

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933	☒
Pre­Effective Amendment No.	□
Post­Effective Amendment No. 23	☒

and/or

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940	☒
Amendment No. 25	☒

Oppenheimer Institutional Government Money Market Fund
(Exact Name of Registrant as Specified in Charter)

6803 South Tucson Way, Centennial, Colorado 80112-3924
(Address of Principal Executive Offices) (Zip Code)

(303) 768-3200

(Registrant’s Telephone Number, including Area Code)

Cynthia Lo Bessette, Esq.

OFI Global Asset Management, Inc.

225 Liberty Street, New York, New York 10281-1008
(Name and Address of Agent for Service)

It is proposed that this filing will become effective (check appropriate box):

☒ immediately upon filing pursuant to paragraph (b)

□ on  pursuant to paragraph (b)

□ 60 days after filing pursuant to paragraph (a)(1)

□ on pursuant to paragraph (a)(1)

□ 75 days after filing pursuant to paragraph (a)(2)

□ on pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

□ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all the requirements for effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and State of New York on the 11th day of October, 2018.

	Oppenheimer Institutional Government Money Market Fund
By:	Arthur P. Steinmetz*
	Arthur P. Steinmetz Trustee, President and Principal Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities on the dates indicated:

Signatures	Title	Date
Brian F. Wruble*Brian F. Wruble	Chairman of the Board of Trustees	October 11, 2018
Arthur P. Steinmetz*Arthur P. Steinmetz	Trustee, President and Principal Executive Officer	October 11, 2018
Brian S. Petersen*Brian S. Petersen	Treasurer, Principal Financial & Accounting Officer	October 11, 2018
Beth Ann Brown*Beth Ann Brown	Trustee	October 11, 2018
Edmund P. Giambastiani, Jr.*Edmund P. Giambastiani, Jr.	Trustee	October 11, 2018
Elizabeth Krentzman*Elizabeth Krentzman	Trustee	October 11, 2018
Mary F. Miller*Mary F. Miller	Trustee	October 11, 2018
Joel W. Motley*Joel W. Motley	Trustee	October 11, 2018
Joanne Pace*Joanne Pace	Trustee	October 11, 2018
Daniel S. Vandivort*Daniel S. Vandivort	Trustee	October 11, 2018
*By: /s/ Taylor V. EdwardsTaylor V. Edwards, Attorney-in-Fact

EXHIBIT INDEX

Exhibit No.	Description
Ex-101.INS	XBRL Instance Document
Ex-101.SCH	XBRL Taxonomy Extension Schema Document
Ex-101.CAL	XBRL Taxonomy Extension Calculation Linkbase Document
Ex-101.DEF	XBRL Taxonomy Extension Definition Linkbase
Ex-101.LAB	XBRL Taxonomy Extension Labels Linkbase
Ex-101.PRE	XBRL Taxonomy Extension Presentation Linkbase Document

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	May 31, 2018
Registrant Name	dei_EntityRegistrantName	Oppenheimer Institutional Government Money Market Fund
Central Index Key	dei_EntityCentralIndexKey	0001358587
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Sep. 26, 2018
Document Effective Date	dei_DocumentEffectiveDate	Sep. 27, 2018
Prospectus Date	rr_ProspectusDate	Sep. 27, 2018

XML

Oppenheimer Institutional Government Money Market Fund

<b>The Fund Summary</b>

<b>Investment Objective.</b>

The Fund seeks income consistent with stability of principal.

<b>Fees and Expenses of the Fund.</b>

The table below describes the fees and expenses that you may pay if you buy and hold or redeem shares of the Fund.

<b>Shareholder Fees. </b>

The Fund does not charge any initial sales charge to buy shares or to reinvest dividends. There are no exchange fees or redemption fees and no contingent deferred sales charges.

<b>Annual Fund Operating Expenses</b><br/>(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Oppenheimer Institutional Government Money Market Fund		Class E	Class L	Class P
Management Fees	[1]	0.10%	0.10%	0.10%
Distribution and/or Service (12b-1) Fees	[1]	none	none	0.25%
Other Expenses	[1]	none	0.06%	0.05%
Total Annual Fund Operating Expenses	[1]	0.10%	0.16%	0.40%
Fee Waiver and/or Expense Reimbursement	[1],[2]	none	none	(0.20%)
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement	[1]	0.10%	0.16%	0.20%

<b>Example.</b>

The following Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in a class of shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Any applicable fee waivers and/or expense reimbursements are reflected in the below examples for the first year only. Although your actual costs may be higher or lower, based on these assumptions your expenses would be as follows:

Expense Example - Oppenheimer Institutional Government Money Market Fund - USD ($)	1 Year	3 Years	5 Years	10 Years
Class E	10	32	57	128
Class L	16	52	90	205
Class P	21	108	205	487

<b>Principal Investment Strategies.</b>

The Fund is a money market fund that intends to qualify as a “government money market fund,” in accordance with Rule 2a-7 under the Investment Company Act of 1940, as amended. As a government money market fund, the Fund must invest at least 99.5% of its total assets in cash, government securities and/or repurchase agreements that are “collateralized fully” (i.e., backed by cash or government securities). The securities in which the Fund invests may have fixed, floating or variable interest rates. The Fund may also invest in affiliated and unaffiliated government money market funds. Additionally, as a government money market fund, the Fund is not required to impose a liquidity fee and/or temporary redemption gate if the Fund’s weekly liquid assets fall below 30% of its total assets. While the Fund’s Board may elect to subject the Fund to liquidity fee and gate requirements in the future, it has not elected to do so at this time.

Under normal market conditions, the Fund invests at least 80% of its net assets (plus borrowings, if any, for investment purposes) in government securities and repurchase agreements that are collateralized by government securities. The 80% investment policy is a non-fundamental investment policy and will not be changed without 60 days’ advance notice to shareholders.

<b>Principal Risks </b>.

All investments carry risks to some degree. The Fund’s investments are subject to changes in their value from a number of factors. However, the Fund’s investments must meet the special rules under Federal law for money market funds. Those requirements include maintaining high credit quality, a short average maturity and diversification of the Fund’s investments among issuers. Those provisions are designed to help minimize credit risks, to reduce the effects of changes in prevailing interest rates and to reduce the effect on the Fund’s portfolio of a default by any one issuer. Since income on short-term securities tends to be lower than income on longer-term debt securities, the Fund’s yield will likely be lower than the yield on longer-term fixed-income funds.

Even so, there are risks that an issuer of an obligation that the Fund holds might have its credit rating downgraded or might default on its obligations, or that interest rates might rise sharply, causing the value of the Fund’s investments to fall. Also, there is the risk that the value of your investment could be eroded over time by the effects of inflation, or that poor security selection could cause the Fund to underperform other funds that have a similar objective. If there is an unexpectedly high demand for the redemption of Fund shares, the Fund might need to sell portfolio securities prior to their maturity, possibly at a loss. As a result, there is a risk that the Fund’s shares could fall below $1.00 per share.

You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time.

Government Securities Risk. The Fund invests in securities issued or guaranteed by the U.S. government or its agencies and instrumentalities (such as securities issued by the Government National Mortgage Association (“Ginnie Mae”), the Federal National Mortgage Association (“Fannie Mae”), the Federal Home Loan Mortgage Corporation (“Freddie Mac”) or other Government Sponsored Enterprises (GSEs)). U.S. government securities are subject to market risk, interest rate risk and credit risk. U.S. government securities include zero coupon securities, which tend to be subject to greater market risk than interest-paying securities of similar maturities.

Risks of Repurchase Agreements. In a repurchase transaction, the Fund buys a security and simultaneously sells it back to the vendor for delivery at a future date. If the seller fails to pay the repurchase price on the delivery date, the Fund may incur costs in disposing of the collateral and may experience losses if there is any delay in its ability to do so. If the default on the part of the seller is due to its bankruptcy, the Fund’s ability to liquidate the collateral may be delayed or limited.

Risks of Money Market Instruments. Money market instruments may be subject to interest rate risk, credit risk, extension risk, reinvestment risk, prepayment risk, and event risk. Interest rate risk is the risk that when prevailing interest rates fall, the values of already-issued debt securities generally rise; and when prevailing interest rates rise, the values of already-issued debt securities generally fall, and they may be worth less than the amount the Fund paid for them. When interest rates change, the values of longer-term debt securities usually change more than the values of shorter-term debt securities. Risks associated with rising interest rates are heightened given that interest rates in the U.S. are near historic lows. Credit risk is the risk that the issuer of a security might not make interest and principal payments on the security as they become due. If an issuer fails to pay interest or repay principal, the Fund’s income or share value might be reduced. Adverse news about an issuer or a downgrade in an issuer’s credit rating, for any reason, can also reduce the market value of the issuer’s securities. Extension risk is the risk that an increase in interest rates could cause principal payments on a debt security to be repaid at a slower rate than expected. Extension risk is particularly prevalent for a callable security where an increase in interest rates could result in the issuer of that security choosing not to redeem the security as anticipated on the security’s call date. Such a decision by the issuer could have the effect of lengthening the debt security’s expected maturity, making it more vulnerable to interest rate risk and reducing its market value. Reinvestment risk is the risk that when interest rates fall the Fund may be required to reinvest the proceeds from a security’s sale or redemption at a lower interest rate. Callable bonds are generally subject to greater reinvestment risk than non-callable bonds. Prepayment risk is the risk that the issuer may redeem the security prior to the expected maturity or that borrowers may repay the loans that underlie these securities more quickly than expected, thereby causing the issuer of the security to repay the principal prior to the expected maturity. The Fund may need to reinvest the proceeds at a lower interest rate, reducing its income. Event risk is the risk that an issuer could be subject to an event, such as a buyout or debt restructuring, that interferes with its ability to make timely interest and principal payments and cause the value of its debt securities to fall.

Fixed-Income Market Risks. The fixed-income securities market can be susceptible to increases in volatility and decreases in liquidity. Liquidity may decline unpredictably in response to overall economic conditions or credit tightening. During times of reduced market liquidity, the Fund may not be able to readily sell securities at the prices at which they are carried on the Fund’s books and could experience a loss. If the Fund needed to sell large blocks of securities to meet shareholder redemption requests or to raise cash, those sales could further reduce the securities’ prices, particularly for lower-rated and unrated securities. An unexpected increase in redemptions by Fund shareholders, (including requests from shareholders who may own a significant percentage of the Fund’s shares) which may be triggered by general market turmoil or an increase in interest rates, as well as other adverse market and economic developments, could cause the Fund to sell its holdings at a loss or at undesirable prices and adversely affect the Fund’s share price and increase the Fund’s liquidity risk, Fund expenses and/or taxable distributions. As of the date of this prospectus, interest rates in the U.S. are near historically low levels, increasing the exposure of bond investors to the risks associated with rising interest rates.

Economic and other market developments can adversely affect fixed-income securities markets in the United States, Europe and elsewhere. At times, participants in debt securities markets may develop concerns about the ability of certain issuers of debt securities to make timely principal and interest payments, or they may develop concerns about the ability of financial institutions that make markets in certain debt securities to facilitate an orderly market. Those concerns may impact the market price or value of those debt securities and may cause increased volatility in those debt securities or debt securities markets. Under some circumstances, as was the case during the latter half of 2008 and early 2009, those concerns could cause reduced liquidity in certain debt securities markets. A lack of liquidity or other adverse credit market conditions may hamper the Fund’s ability to sell the debt securities in which it invests or to find and purchase suitable debt instruments.

Net Asset Value Risk. There is no assurance that the Fund will maintain a net asset value of $1.00 per share on a continuous basis. Additionally, there can be no assurance that the Fund’s affiliates will purchase distressed assets from the Fund, make capital infusions, enter into capital support agreements or take other actions to ensure that the Fund maintains a net asset value of $1.00 per share. The failure of any money market fund to maintain a stable net asset value, could place increased redemption pressure on other money market funds, including the Fund.

Risks Relating to Investments By Other Oppenheimer Funds. Other Oppenheimer funds may invest all or a portion of their uninvested cash in shares of the Fund and may own a significant portion of the Fund’s shares. These Oppenheimer funds may increase or reduce the amount of their investments in the Fund frequently, particularly under volatile market conditions, and in certain circumstances, such activity could require the Fund to purchase or sell portfolio securities, which may increase the Fund’s transaction costs and/or reduce its performance.

Regulatory Risk. Changes to the monetary policy by the Federal Reserve or other regulatory actions could expose fixed income and related markets to heightened volatility, interest rate sensitivity and reduced liquidity, which may impact the Fund’s operations, universe of potential investment options, and return potential.

Yield Risk. During periods of extremely low short-term interest rates, the Fund may not be able to maintain a positive yield. The rate of the Fund’s income will vary from day to day, generally reflecting changes in short-term interest rates and in the fixed-income securities market. There is no assurance that the Fund will achieve its investment objective.

Cash Risk. As a government money market fund, the Fund will likely hold some of its assets in cash, which may negatively affect the Fund’s performance. Maintaining cash positions may also subject the Fund to additional risks and costs, such as increased exposure to the custodian bank holding the assets and any fees imposed for large cash balances.

Risk of Investing in Floating and Variable Rate Obligations. Some fixed-income securities have variable or floating interest rates that provide for a periodic adjustment in the interest rate paid on the securities. The rate adjustment intervals may be regular and range from daily up to annually, or may be based on an event, such as a change in the stated prevailing market rate. Floating and variable rate securities may be subject to greater liquidity risk than other debt securities, meaning that there may be limitations on the Fund’s ability to sell the securities at any given time. Such securities also may lose value.

Large Shareholder Transactions Risk. Large transactions by shareholders can impact the Fund’s expense ratio, yield and potentially its net asset value. A large redemption of Fund shares by a large shareholder may have a negative effect on the Fund’s net asset value and yield, as the Fund may be forced to sell a large portion of its portfolio holdings at an inopportune time. A large redemption of Fund shares may also result in an increase in the Fund’s expense ratio, since a large redemption may result in the Fund’s current expenses being allocated over a smaller asset base. In order to be able to meet reasonably foreseeable requests for redemptions of Fund shares, the Fund may be required to consider factors that could affect the Fund’s liquidity needs, including characteristics of the Fund’s investors and their likely redemptions. This may require the Fund to maintain sufficiently liquid assets in lower-yielding securities that are easier to sell, which may have a negative impact on the Fund’s yield. Similarly, a large purchase of Fund shares by a large shareholder may have a negative effect on the Fund’s yield, as the Fund may be unable to deploy a larger cash position into new investments as quickly as it could with a smaller cash position. Large transactions may also increase transaction costs.

Who Is the Fund Designed For? The Fund is designed for institutional investors who want to earn income at money market rates while preserving the value of their investment and maintaining easy access to their investment through wire redemption privileges. The Fund will invest in cash, government securities and/or repurchase agreements that are collateralized fully by cash or government securities to seek income consistent with stability of principal and to try to maintain a stable share price of $1.00. Income on short-term securities tends to be lower than income on longer-term debt securities, so the Fund’s yield will likely be lower than the yield on longer-term fixed-income funds. The Fund does not invest for the purpose of seeking capital appreciation or gains and is not a complete investment program.

You could lose money by investing in the Fund. An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the Fund. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time.

<b>The Fund’s Past Performance.</b>

The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance for Class E Shares from calendar year to calendar year and by showing the Fund’s average annual returns for the periods of time shown in the table. The Fund’s past investment performance is not necessarily an indication of how the Fund will perform in the future. Sales charges and taxes are not reflected in the bar chart and if those charges were included, returns would be less than those shown. Prior to September 28, 2016, the Fund was operated as a prime money market fund. Since September 28, 2016, the Fund has operated as a government money market fund and, as such, must invest at least 99.5% of its total assets in cash, government securities and/or repurchase agreements that are “collateralized fully” (i.e., backed by cash or government securities). More recent performance information is available by calling the toll-free number on the back of this prospectus and on the Fund’s website at: https://www.oppenheimerfunds.com/fund/InstitutionalGovernmentMoneyMarketFund

Sales charges and taxes are not included and the returns would be lower if they were. During the period shown, the highest return for a calendar quarter was 1.04% (1st Qtr 08) and the lowest return for the most recent calendar quarter was 0.02% (4th Qtr 14). For the period from January 1, 2018 to June 30, 2018 the return before sales charges and taxes was 0.75%.

<b>Average Annual Total Returns </b>for the periods ended December 31, 2017

Average Annual Total Returns - Oppenheimer Institutional Government Money Market Fund	1 Year	5 Years	10 Years	Inception Date
Class E Shares	0.82%	0.31%	0.59%	Oct. 03, 2006
Class L Shares	0.76%	0.26%	0.54%	Oct. 03, 2006
Class P Shares	0.76%	0.24%	0.51%	May 21, 2007

The Fund’s performance reflects reinvestment of all dividends and capital gains distributions.

The total returns are not the Fund’s current yield. The Fund’s yield more closely reflects the Fund’s current earnings. To obtain the Fund’s current 7-day yield information, please call the Transfer Agent toll-free at 1.800.988.8287.

[1] Expenses have been restated to reflect current fees.

[2] The Fund’s Distributor has contractually agreed to waive the fees payable under the Service Plan for Class P shares so that those fees are limited to 0.05% of the average annual net assets for Class P shares of the Fund. This fee waiver may not be amended or withdrawn for one year from the date of this prospectus.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	Oppenheimer Institutional Government Money Market Fund
Prospectus Date	rr_ProspectusDate	Sep. 27, 2018
Oppenheimer Institutional Government Money Market Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>The Fund Summary</b>
Objective [Heading]	rr_ObjectiveHeading	<b>Investment Objective.</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks income consistent with stability of principal.
Expense [Heading]	rr_ExpenseHeading	<b>Fees and Expenses of the Fund.</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold or redeem shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	<b>Shareholder Fees. </b>
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Fund Operating Expenses</b><br/>(expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	one year from the date of this prospectus
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Expenses have been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>Example.</b>
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in a class of shares of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Any applicable fee waivers and/or expense reimbursements are reflected in the below examples for the first year only. Although your actual costs may be higher or lower, based on these assumptions your expenses would be as follows:
Strategy [Heading]	rr_StrategyHeading	<b>Principal Investment Strategies.</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund is a money market fund that intends to qualify as a “government money market fund,” in accordance with Rule 2a-7 under the Investment Company Act of 1940, as amended. As a government money market fund, the Fund must invest at least 99.5% of its total assets in cash, government securities and/or repurchase agreements that are “collateralized fully” (i.e., backed by cash or government securities). The securities in which the Fund invests may have fixed, floating or variable interest rates. The Fund may also invest in affiliated and unaffiliated government money market funds. Additionally, as a government money market fund, the Fund is not required to impose a liquidity fee and/or temporary redemption gate if the Fund’s weekly liquid assets fall below 30% of its total assets. While the Fund’s Board may elect to subject the Fund to liquidity fee and gate requirements in the future, it has not elected to do so at this time. Under normal market conditions, the Fund invests at least 80% of its net assets (plus borrowings, if any, for investment purposes) in government securities and repurchase agreements that are collateralized by government securities. The 80% investment policy is a non-fundamental investment policy and will not be changed without 60 days’ advance notice to shareholders.
Risk [Heading]	rr_RiskHeading	<b>Principal Risks </b>.
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	All investments carry risks to some degree. The Fund’s investments are subject to changes in their value from a number of factors. However, the Fund’s investments must meet the special rules under Federal law for money market funds. Those requirements include maintaining high credit quality, a short average maturity and diversification of the Fund’s investments among issuers. Those provisions are designed to help minimize credit risks, to reduce the effects of changes in prevailing interest rates and to reduce the effect on the Fund’s portfolio of a default by any one issuer. Since income on short-term securities tends to be lower than income on longer-term debt securities, the Fund’s yield will likely be lower than the yield on longer-term fixed-income funds. Even so, there are risks that an issuer of an obligation that the Fund holds might have its credit rating downgraded or might default on its obligations, or that interest rates might rise sharply, causing the value of the Fund’s investments to fall. Also, there is the risk that the value of your investment could be eroded over time by the effects of inflation, or that poor security selection could cause the Fund to underperform other funds that have a similar objective. If there is an unexpectedly high demand for the redemption of Fund shares, the Fund might need to sell portfolio securities prior to their maturity, possibly at a loss. As a result, there is a risk that the Fund’s shares could fall below $1.00 per share. You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time. Government Securities Risk. The Fund invests in securities issued or guaranteed by the U.S. government or its agencies and instrumentalities (such as securities issued by the Government National Mortgage Association (“Ginnie Mae”), the Federal National Mortgage Association (“Fannie Mae”), the Federal Home Loan Mortgage Corporation (“Freddie Mac”) or other Government Sponsored Enterprises (GSEs)). U.S. government securities are subject to market risk, interest rate risk and credit risk. U.S. government securities include zero coupon securities, which tend to be subject to greater market risk than interest-paying securities of similar maturities. Risks of Repurchase Agreements. In a repurchase transaction, the Fund buys a security and simultaneously sells it back to the vendor for delivery at a future date. If the seller fails to pay the repurchase price on the delivery date, the Fund may incur costs in disposing of the collateral and may experience losses if there is any delay in its ability to do so. If the default on the part of the seller is due to its bankruptcy, the Fund’s ability to liquidate the collateral may be delayed or limited. Risks of Money Market Instruments. Money market instruments may be subject to interest rate risk, credit risk, extension risk, reinvestment risk, prepayment risk, and event risk. Interest rate risk is the risk that when prevailing interest rates fall, the values of already-issued debt securities generally rise; and when prevailing interest rates rise, the values of already-issued debt securities generally fall, and they may be worth less than the amount the Fund paid for them. When interest rates change, the values of longer-term debt securities usually change more than the values of shorter-term debt securities. Risks associated with rising interest rates are heightened given that interest rates in the U.S. are near historic lows. Credit risk is the risk that the issuer of a security might not make interest and principal payments on the security as they become due. If an issuer fails to pay interest or repay principal, the Fund’s income or share value might be reduced. Adverse news about an issuer or a downgrade in an issuer’s credit rating, for any reason, can also reduce the market value of the issuer’s securities. Extension risk is the risk that an increase in interest rates could cause principal payments on a debt security to be repaid at a slower rate than expected. Extension risk is particularly prevalent for a callable security where an increase in interest rates could result in the issuer of that security choosing not to redeem the security as anticipated on the security’s call date. Such a decision by the issuer could have the effect of lengthening the debt security’s expected maturity, making it more vulnerable to interest rate risk and reducing its market value. Reinvestment risk is the risk that when interest rates fall the Fund may be required to reinvest the proceeds from a security’s sale or redemption at a lower interest rate. Callable bonds are generally subject to greater reinvestment risk than non-callable bonds. Prepayment risk is the risk that the issuer may redeem the security prior to the expected maturity or that borrowers may repay the loans that underlie these securities more quickly than expected, thereby causing the issuer of the security to repay the principal prior to the expected maturity. The Fund may need to reinvest the proceeds at a lower interest rate, reducing its income. Event risk is the risk that an issuer could be subject to an event, such as a buyout or debt restructuring, that interferes with its ability to make timely interest and principal payments and cause the value of its debt securities to fall. Fixed-Income Market Risks. The fixed-income securities market can be susceptible to increases in volatility and decreases in liquidity. Liquidity may decline unpredictably in response to overall economic conditions or credit tightening. During times of reduced market liquidity, the Fund may not be able to readily sell securities at the prices at which they are carried on the Fund’s books and could experience a loss. If the Fund needed to sell large blocks of securities to meet shareholder redemption requests or to raise cash, those sales could further reduce the securities’ prices, particularly for lower-rated and unrated securities. An unexpected increase in redemptions by Fund shareholders, (including requests from shareholders who may own a significant percentage of the Fund’s shares) which may be triggered by general market turmoil or an increase in interest rates, as well as other adverse market and economic developments, could cause the Fund to sell its holdings at a loss or at undesirable prices and adversely affect the Fund’s share price and increase the Fund’s liquidity risk, Fund expenses and/or taxable distributions. As of the date of this prospectus, interest rates in the U.S. are near historically low levels, increasing the exposure of bond investors to the risks associated with rising interest rates. Economic and other market developments can adversely affect fixed-income securities markets in the United States, Europe and elsewhere. At times, participants in debt securities markets may develop concerns about the ability of certain issuers of debt securities to make timely principal and interest payments, or they may develop concerns about the ability of financial institutions that make markets in certain debt securities to facilitate an orderly market. Those concerns may impact the market price or value of those debt securities and may cause increased volatility in those debt securities or debt securities markets. Under some circumstances, as was the case during the latter half of 2008 and early 2009, those concerns could cause reduced liquidity in certain debt securities markets. A lack of liquidity or other adverse credit market conditions may hamper the Fund’s ability to sell the debt securities in which it invests or to find and purchase suitable debt instruments. Net Asset Value Risk. There is no assurance that the Fund will maintain a net asset value of $1.00 per share on a continuous basis. Additionally, there can be no assurance that the Fund’s affiliates will purchase distressed assets from the Fund, make capital infusions, enter into capital support agreements or take other actions to ensure that the Fund maintains a net asset value of $1.00 per share. The failure of any money market fund to maintain a stable net asset value, could place increased redemption pressure on other money market funds, including the Fund. Risks Relating to Investments By Other Oppenheimer Funds. Other Oppenheimer funds may invest all or a portion of their uninvested cash in shares of the Fund and may own a significant portion of the Fund’s shares. These Oppenheimer funds may increase or reduce the amount of their investments in the Fund frequently, particularly under volatile market conditions, and in certain circumstances, such activity could require the Fund to purchase or sell portfolio securities, which may increase the Fund’s transaction costs and/or reduce its performance. Regulatory Risk. Changes to the monetary policy by the Federal Reserve or other regulatory actions could expose fixed income and related markets to heightened volatility, interest rate sensitivity and reduced liquidity, which may impact the Fund’s operations, universe of potential investment options, and return potential. Yield Risk. During periods of extremely low short-term interest rates, the Fund may not be able to maintain a positive yield. The rate of the Fund’s income will vary from day to day, generally reflecting changes in short-term interest rates and in the fixed-income securities market. There is no assurance that the Fund will achieve its investment objective. Cash Risk. As a government money market fund, the Fund will likely hold some of its assets in cash, which may negatively affect the Fund’s performance. Maintaining cash positions may also subject the Fund to additional risks and costs, such as increased exposure to the custodian bank holding the assets and any fees imposed for large cash balances. Risk of Investing in Floating and Variable Rate Obligations. Some fixed-income securities have variable or floating interest rates that provide for a periodic adjustment in the interest rate paid on the securities. The rate adjustment intervals may be regular and range from daily up to annually, or may be based on an event, such as a change in the stated prevailing market rate. Floating and variable rate securities may be subject to greater liquidity risk than other debt securities, meaning that there may be limitations on the Fund’s ability to sell the securities at any given time. Such securities also may lose value. Large Shareholder Transactions Risk. Large transactions by shareholders can impact the Fund’s expense ratio, yield and potentially its net asset value. A large redemption of Fund shares by a large shareholder may have a negative effect on the Fund’s net asset value and yield, as the Fund may be forced to sell a large portion of its portfolio holdings at an inopportune time. A large redemption of Fund shares may also result in an increase in the Fund’s expense ratio, since a large redemption may result in the Fund’s current expenses being allocated over a smaller asset base. In order to be able to meet reasonably foreseeable requests for redemptions of Fund shares, the Fund may be required to consider factors that could affect the Fund’s liquidity needs, including characteristics of the Fund’s investors and their likely redemptions. This may require the Fund to maintain sufficiently liquid assets in lower-yielding securities that are easier to sell, which may have a negative impact on the Fund’s yield. Similarly, a large purchase of Fund shares by a large shareholder may have a negative effect on the Fund’s yield, as the Fund may be unable to deploy a larger cash position into new investments as quickly as it could with a smaller cash position. Large transactions may also increase transaction costs. Who Is the Fund Designed For? The Fund is designed for institutional investors who want to earn income at money market rates while preserving the value of their investment and maintaining easy access to their investment through wire redemption privileges. The Fund will invest in cash, government securities and/or repurchase agreements that are collateralized fully by cash or government securities to seek income consistent with stability of principal and to try to maintain a stable share price of $1.00. Income on short-term securities tends to be lower than income on longer-term debt securities, so the Fund’s yield will likely be lower than the yield on longer-term fixed-income funds. The Fund does not invest for the purpose of seeking capital appreciation or gains and is not a complete investment program. You could lose money by investing in the Fund. An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the Fund. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the Fund.
Risk Money Market Fund [Text]	rr_RiskMoneyMarketFund	Although the Fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the Fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>The Fund’s Past Performance.</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance for Class E Shares from calendar year to calendar year and by showing the Fund’s average annual returns for the periods of time shown in the table. The Fund’s past investment performance is not necessarily an indication of how the Fund will perform in the future. Sales charges and taxes are not reflected in the bar chart and if those charges were included, returns would be less than those shown. Prior to September 28, 2016, the Fund was operated as a prime money market fund. Since September 28, 2016, the Fund has operated as a government money market fund and, as such, must invest at least 99.5% of its total assets in cash, government securities and/or repurchase agreements that are “collateralized fully” (i.e., backed by cash or government securities). More recent performance information is available by calling the toll-free number on the back of this prospectus and on the Fund’s website at: https://www.oppenheimerfunds.com/fund/InstitutionalGovernmentMoneyMarketFund
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and table below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance for Class E Shares from calendar year to calendar year and by showing the Fund’s average annual returns for the periods of time shown in the table.
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	https://www.oppenheimerfunds.com/fund/InstitutionalGovernmentMoneyMarketFund
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past investment performance is not necessarily an indication of how the Fund will perform in the future.
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges and taxes are not reflected in the bar chart and if those charges were included, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Sales charges and taxes are not included and the returns would be lower if they were. During the period shown, the highest return for a calendar quarter was 1.04% (1st Qtr 08) and the lowest return for the most recent calendar quarter was 0.02% (4th Qtr 14). For the period from January 1, 2018 to June 30, 2018 the return before sales charges and taxes was 0.75%.
Performance Table Heading	rr_PerformanceTableHeading	<b>Average Annual Total Returns </b>for the periods ended December 31, 2017
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	The Fund’s performance reflects reinvestment of all dividends and capital gains distributions. The total returns are not the Fund’s current yield. The Fund’s yield more closely reflects the Fund’s current earnings. To obtain the Fund’s current 7-day yield information, please call the Transfer Agent toll-free at 1.800.988.8287.
Money Market Seven Day Yield, Caption [Text]	rr_MoneyMarketSevenDayYieldCaption	<i><b>To obtain the Fund’s current 7-day yield information, please call the Transfer Agent toll-free at 1.800.988.8287.</b></i>
Money Market Seven Day Yield Phone	rr_MoneyMarketSevenDayYieldPhone	<i><b>1.800.988.8287</b></i>
Oppenheimer Institutional Government Money Market Fund | Class E
Risk/Return:	rr_RiskReturnAbstract
Management Fees	rr_ManagementFeesOverAssets	0.10%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none	[1]
Other Expenses	rr_OtherExpensesOverAssets	none	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.10%	[1]
Fee Waiver and/or Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none	[1],[2]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement	rr_NetExpensesOverAssets	0.10%	[1]
1 Year	rr_ExpenseExampleYear01	$ 10
3 Years	rr_ExpenseExampleYear03	32
5 Years	rr_ExpenseExampleYear05	57
10 Years	rr_ExpenseExampleYear10	$ 128
2008	rr_AnnualReturn2008	3.14%
2009	rr_AnnualReturn2009	0.58%
2010	rr_AnnualReturn2010	0.23%
2011	rr_AnnualReturn2011	0.18%
2012	rr_AnnualReturn2012	0.20%
2013	rr_AnnualReturn2013	0.12%
2014	rr_AnnualReturn2014	0.09%
2015	rr_AnnualReturn2015	0.16%
2016	rr_AnnualReturn2016	0.40%
2017	rr_AnnualReturn2017	0.82%
Year to Date Return, Label	rr_YearToDateReturnLabel	For the period from January 1, 2018 to June 30, 2018
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.75%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	1.04%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2014
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	0.02%
1 Year	rr_AverageAnnualReturnYear01	0.82%
5 Years	rr_AverageAnnualReturnYear05	0.31%
10 Years	rr_AverageAnnualReturnYear10	0.59%
Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 03, 2006
Oppenheimer Institutional Government Money Market Fund | Class L
Risk/Return:	rr_RiskReturnAbstract
Management Fees	rr_ManagementFeesOverAssets	0.10%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none	[1]
Other Expenses	rr_OtherExpensesOverAssets	0.06%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.16%	[1]
Fee Waiver and/or Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	none	[1],[2]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement	rr_NetExpensesOverAssets	0.16%	[1]
1 Year	rr_ExpenseExampleYear01	$ 16
3 Years	rr_ExpenseExampleYear03	52
5 Years	rr_ExpenseExampleYear05	90
10 Years	rr_ExpenseExampleYear10	$ 205
1 Year	rr_AverageAnnualReturnYear01	0.76%
5 Years	rr_AverageAnnualReturnYear05	0.26%
10 Years	rr_AverageAnnualReturnYear10	0.54%
Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 03, 2006
Oppenheimer Institutional Government Money Market Fund | Class P
Risk/Return:	rr_RiskReturnAbstract
Management Fees	rr_ManagementFeesOverAssets	0.10%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%	[1]
Other Expenses	rr_OtherExpensesOverAssets	0.05%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.40%	[1]
Fee Waiver and/or Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.20%)	[1],[2]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement	rr_NetExpensesOverAssets	0.20%	[1]
1 Year	rr_ExpenseExampleYear01	$ 21
3 Years	rr_ExpenseExampleYear03	108
5 Years	rr_ExpenseExampleYear05	205
10 Years	rr_ExpenseExampleYear10	$ 487
1 Year	rr_AverageAnnualReturnYear01	0.76%
5 Years	rr_AverageAnnualReturnYear05	0.24%
10 Years	rr_AverageAnnualReturnYear10	0.51%
Inception Date	rr_AverageAnnualReturnInceptionDate	May 21, 2007

[1] Expenses have been restated to reflect current fees.

[2] The Fund’s Distributor has contractually agreed to waive the fees payable under the Service Plan for Class P shares so that those fees are limited to 0.05% of the average annual net assets for Class P shares of the Fund. This fee waiver may not be amended or withdrawn for one year from the date of this prospectus.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	Oppenheimer Institutional Government Money Market Fund
Prospectus Date	rr_ProspectusDate	Sep. 27, 2018
Document Creation Date	dei_DocumentCreationDate	Sep. 26, 2018