Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions - Fair Value of Derivatives (Details)	12 Months Ended
Dec. 31, 2022 USD ($) item	Dec. 31, 2021 USD ($) item
Two financial institutions
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Cash collateral pledged with financial institutions, dollars	$ 11,200,000
Number of financial institutions in which cash collateral pledged \| item	2
One financial institutions
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Cash collateral pledged with financial institutions, dollars		$ 17,200,000
Number of financial institutions in which cash collateral pledged \| item		1
Interest Rate Swap [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Investment securities pledged with financial institutions, dollars	$ 0	$ 0
Number of financial institutions in which cash collateral pledged	5,300,000	180,000,000
Designated as Hedging Instrument
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative assets designated as hedging instruments, fair value	$ 2,737,000	$ 808,000
Derivative liabilities designated as hedging instruments, fair value	$ 8,610,000	$ 4,102,000
Designated as Hedging Instrument \| Interest Rate Products \| OSBC Affiliates
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	4	2
Notional amount	$ 275,774,000	$ 75,774,000
Derivative assets designated as hedging instruments, fair value	2,737,000	808,000
Derivative liabilities designated as hedging instruments, fair value	8,610,000	4,102,000
Designated as Hedging Instrument \| Interest Rate Swap [Member] \| Cash flow hedges of certain variable rate commercial and commercial real estate loan pools
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional amount	250,000,000.0	50,000,000.0
Designated as Hedging Instrument \| Interest Rate Swap [Member] \| Cash flow hedge of junior subordinated debentures and was executed to pay fixed and receive variable rate cash flows
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional amount	25,800,000	25,800,000
Not Designated as Hedging Instrument
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative assets designated as hedging instruments, fair value	3,855,000	3,194,000
Derivative liabilities designated as hedging instruments, fair value	$ 3,655,000	$ 2,707,000
Not Designated as Hedging Instrument \| Interest Rate Products \| Commercial Loan Customers
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	21	26
Notional amount	$ 110,647,000	$ 165,005,000
Derivative assets designated as hedging instruments, fair value	3,654,000	2,686,000
Derivative liabilities designated as hedging instruments, fair value	3,654,000	2,686,000
Not Designated as Hedging Instrument \| Interest Rate Swap [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional amount	110,600,000	165,000,000.0
Not Designated as Hedging Instrument \| Commitments.
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional amount	$ 5,300,000	$ 34,400,000
Not Designated as Hedging Instrument \| Interest Rate Lock Commitments and Forward Contracts.
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	28	87
Notional amount	$ 5,298,000	$ 34,414,000
Derivative assets designated as hedging instruments, fair value	$ 76,000	$ 508,000
Not Designated as Hedging Instrument \| Other Contracts
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Derivative, number of transactions \| item	4	3
Notional amount	$ 43,699,000	$ 17,173,000
Derivative assets designated as hedging instruments, fair value	125,000
Derivative liabilities designated as hedging instruments, fair value	1,000	21,000
Not Designated as Hedging Instrument \| Forward Contracts [Member]
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Notional amount	2,750,000	$ 20,000,000
Interest Income
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Amount to be reclassified as an decrease to interest expense during the next twelve months	5,200,000
Interest Expense
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions
Amount to be reclassified as an decrease to interest expense during the next twelve months	$ 563,000

Represents the number of financial institutions in which cash collateral pledged.

No definition available.

Name:	osbc_NumberOfFinancialInstitutionsInWhichCashCollateralPledged
Namespace Prefix:	osbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.1)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 2: 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),2)
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

+ Details

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

- Definition

Fair value of all asset derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959

+ Details

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

- Definition

Fair value of all liability derivatives designated as cash flow hedging instruments.

+ References

+ Details

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

- Definition

Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

Name:	us-gaap_InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=osbc_CashFlowHedgesOfCertainVariableRateCommercialAndCommercialRealEstateLoanPoolsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=osbc_CashFlowHedgeOfJuniorSubordinatedDebenturesAndWasExecutedToPayFixedAndReceiveVariableRateCashFlowsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=osbc_DerivativeCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=osbc_InterestRateLockCommitmentsAndForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: