Number of financing receivable modified during period for debtor experiencing financial difficulty.

No definition available.

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

- Definition

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amortized cost excluding accrued interest, of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 44
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-44

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=osbc_PaymentModificationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=osbc_IncreasedMonthlyPaymentUntilMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=osbc_ReductionOfMonthlyPaymentUntilMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=osbc_InterestOnlyPaymentsDuringForbearanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=osbc_SinglePaymentAtMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Credit Losses on Loans - TDR (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) loan	Sep. 30, 2023 USD ($) loan	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Troubled debt restructurings - modified during the period
Number of payment modification			13	15
Total Loans modified \| $	$ 37,004	$ 26,596	$ 41,172	$ 43,003	$ 42,865	$ 43,003
Payment default \| $			$ 0	$ 0
Payment modification
Troubled debt restructurings - modified during the period
Number of payment modification			4	2
Increase of monthly payment until maturity
Troubled debt restructurings - modified during the period
Number of payment modification			1
Reduction of monthly payment until maturity
Troubled debt restructurings - modified during the period
Number of payment modification			1
Interest-only payments during forbearance
Troubled debt restructurings - modified during the period
Number of payment modification			2
Single payment at maturity
Troubled debt restructurings - modified during the period
Number of payment modification				1
Commercial
Troubled debt restructurings - modified during the period
Total Loans modified \| $	$ 3,794	$ 864	$ 4,041	$ 2,692	$ 5,536	$ 2,692