Securities - Amortized Cost and Fair Value (Details) $ in Thousands	12 Months Ended
Dec. 31, 2022 USD ($) item	Dec. 31, 2021 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 1,662,819	$ 1,678,160
Gross Unrealized Gains	370	20,587
Gross Unrealized Losses	(123,830)	(5,115)
Securities available-for-sale	1,539,359	1,693,632
Accrued interest receivable	6,800	4,300
FHLB and FRB Stock
FHLBC stock	5,600	7,100
FRBC stock	14,900	6,200
Securities pledged to secure deposits and for other purposes	547,800	501,300
Other disclosures
Securities available-for-sale	$ 1,539,359	1,693,632
Number of securities issued from originators	0
Number of originators \| item	1
Minimum percentage of securities investment	10.00%
US government and agencies debt securities, percentage of guarantee	100.00%
US government and agencies debt securities, threshold percentage of guarantee for allowance of credit loss	100.00%
Reimbursement requests greater than 5 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	85
Percent of insured loan	5
Reimbursement requests greater than 9 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	75
Percent of insured loan	9
U.S. Treasury
Securities Available-for-Sale
Amortized Cost	$ 224,054	202,251
Gross Unrealized Gains		125
Gross Unrealized Losses	(11,925)	(37)
Securities available-for-sale	212,129	202,339
Other disclosures
Securities available-for-sale	212,129	202,339
U.S. government agencies
Securities Available-for-Sale
Amortized Cost	61,178	62,587
Gross Unrealized Losses	(5,130)	(699)
Securities available-for-sale	56,048	61,888
Other disclosures
Securities available-for-sale	56,048	61,888
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	140,588	172,016
Gross Unrealized Gains		856
Gross Unrealized Losses	(15,598)	(570)
Securities available-for-sale	124,990	172,302
Other disclosures
Securities available-for-sale	124,990	172,302
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	239,999	241,937
Gross Unrealized Gains	363	16,344
Gross Unrealized Losses	(14,234)	(672)
Securities available-for-sale	226,128	257,609
Other disclosures
Securities available-for-sale	226,128	257,609
Corporate bonds
Securities Available-for-Sale
Amortized Cost	10,000	10,000
Gross Unrealized Losses	(378)	(113)
Securities available-for-sale	9,622	9,887
Other disclosures
Securities available-for-sale	9,622	9,887
Collateralized mortgage obligations
Securities Available-for-Sale
Amortized Cost	596,336	673,238
Gross Unrealized Gains	1	2,014
Gross Unrealized Losses	(62,569)	(2,285)
Securities available-for-sale	533,768	672,967
Other disclosures
Securities available-for-sale	533,768	672,967
Asset-backed Securities
Securities Available-for-Sale
Amortized Cost	210,388	236,293
Gross Unrealized Gains	6	1,245
Gross Unrealized Losses	(8,466)	(661)
Securities available-for-sale	201,928	236,877
Other disclosures
Securities available-for-sale	201,928	236,877
FFEL
Securities Available-for-Sale
Securities available-for-sale	160,800
Other disclosures
Securities available-for-sale	$ 160,800
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	97
Collateralized loan obligations.
Securities Available-for-Sale
Amortized Cost	$ 180,276	79,838
Gross Unrealized Gains		3
Gross Unrealized Losses	(5,530)	(78)
Securities available-for-sale	174,746	79,763
Other disclosures
Securities available-for-sale	$ 174,746	$ 79,763
Loan receivables \| GCO Education Loan Funding Corp \| Customer Concentration Risk
Other disclosures
Percentage of concentration risk	1.00%
Minimum
Other disclosures
Debt securities, available-for-sale, allowance for credit loss, percent of market value loss	5.00%
Debt securities, available-for-sale, allowance for credit loss, market value loss	$ 200,000

The market value loss of debt securities available for sale, for evaluation of allowance for credit loss.

No definition available.

Name:	osbc_DebtSecuritiesAvailableForSaleAllowanceForCreditLossMarketValueLoss
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The market value loss of debt securities available for sale, as percent of book value for evaluation of allowance for credit loss.

+ References

No definition available.

+ Details

Name:	osbc_DebtSecuritiesAvailableForSaleAllowanceForCreditLossPercentOfMarketValueLoss
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of securities investment from originators.

+ References

No definition available.

+ Details

Name:	osbc_MinimumPercentageOfSecuritiesInvestmentFromOriginators
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of originators that the company has invested over 10 % of its stockholders equity.

+ References

No definition available.

+ Details

Name:	osbc_NumberOfOriginatorsThatCompanyHasInvestedOverSpecifiedPercentageOfItsStockholdersEquity
Namespace Prefix:	osbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of securities issued from originators.

+ References

No definition available.

+ Details

Name:	osbc_NumberOfSecuritiesIssuedFromOriginators
Namespace Prefix:	osbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percent of insured loans used as base for loans guaranteed by US Department of Education.

+ References

No definition available.

+ Details

Name:	osbc_PercentOfInsuredLoansUsedAsBaseForLoansGuaranteedByUSDepartmentOfEducation
Namespace Prefix:	osbc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects percentage of outstanding principal amount of loans guaranteed by US Department of Education.

+ References

No definition available.

+ Details

Name:	osbc_PercentageOfOutstandingPrincipalAmountOfLoansGuaranteedByUsDepartmentOfEducation
Namespace Prefix:	osbc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of guarantee by Us government and agencies.

+ References

No definition available.

+ Details

Name:	osbc_UsGovernmentAndAgenciesDebtSecuritiesPercentageOfGuarantee
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The threshold percentage of guarantee by Us government and agencies for allowance of credit loss.

+ References

No definition available.

+ Details

Name:	osbc_UsGovernmentAndAgenciesDebtSecuritiesThresholdPercentageOfGuaranteeForAllowanceOfCreditLoss
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(4))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(a)(4))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_AccruedInvestmentIncomeReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI https://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI https://asc.fasb.org/extlink&oid=123594938&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123594938&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI https://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 954
-SubTopic 210
-Section 45
-Paragraph 4
-URI https://asc.fasb.org/extlink&oid=120413173&loc=SL6242262-115580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI https://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 954
-SubTopic 210
-Section 50
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=127002003&loc=SL6242269-115581

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleRestricted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 325
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=123599499&loc=d3e63345-112809

+ Details

Name:	us-gaap_FederalHomeLoanBankStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankStockAndFederalReserveBankStockAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

+ Details

Name:	us-gaap_FederalReserveBankStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_MarketableSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_StatementScenarioAxis=osbc_ReimbursementRequestsGreaterThan5PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=osbc_ReimbursementRequestsGreaterThan9PercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=osbc_FederalFamilyEducationLoanProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_AccountsReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=osbc_GCOEducationLoanFundingCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: