Loans and Allowance for Credit Losses on Loans (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	$ 44,113	$ 53,392	$ 44,264	$ 49,480
Provision for (Release of) Credit Losses	3,950	2,427	7,494	7,079
Charge-offs	6,011	733	9,999	1,755
Recoveries	217	228	510	510
Financing Receivable, Allowance for Credit Loss, Ending Balance	42,269	55,314	42,269	55,314
Net provision expense			7,500
Commercial
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	6,382	11,511	3,998	11,968
Provision for (Release of) Credit Losses	327	319	2,653	(262,000)
Charge-offs	3	380	18	407
Recoveries	22	82	95	233
Financing Receivable, Allowance for Credit Loss, Ending Balance	6,728	11,532	6,728	11,532
Leases
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	2,959	2,766	2,952	2,865
Provision for (Release of) Credit Losses	(900)	(83)	(933)	691,000
Charge-offs	81		81	882
Recoveries		7	40	16
Financing Receivable, Allowance for Credit Loss, Ending Balance	1,978	2,690	1,978	2,690
Commercial real estate - Investor
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	16,270	15,260	17,105	10,674
Provision for (Release of) Credit Losses	6,132	4,822	5,230	9,391,000
Charge-offs	4,580	71	4,596	71
Recoveries	20	20	103	37
Financing Receivable, Allowance for Credit Loss, Ending Balance	17,842	20,031	17,842	20,031
Commercial real estate - Owner occupied
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	10,992	15,576	12,280	15,001
Provision for (Release of) Credit Losses	(2,650)	(2,816)	(70)	(2,243,000)
Charge-offs	1,281	201	5,168	201
Recoveries	119	3	138	5
Financing Receivable, Allowance for Credit Loss, Ending Balance	7,180	12,562	7,180	12,562
Real estate - construction
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	1,097	1,045
Provision for (Release of) Credit Losses	923	134
Financing Receivable, Allowance for Credit Loss, Ending Balance	2,020	1,179	2,020	1,179
Construction
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance			1,038	1,546
Provision for (Release of) Credit Losses			982	(367,000)
Financing Receivable, Allowance for Credit Loss, Ending Balance	2,020	1,179	2,020	1,179
Residential real estate - Investor
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	636	746	669	768
Provision for (Release of) Credit Losses	(30)	(8)	(65)	(49,000)
Recoveries	3	5	5	24
Financing Receivable, Allowance for Credit Loss, Ending Balance	609	743	609	743
Residential real estate - Owner occupied
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	1,660	1,722	1,821	2,046
Provision for (Release of) Credit Losses	(51)	110	(220)	(224,000)
Recoveries	9	36	17	46
Financing Receivable, Allowance for Credit Loss, Ending Balance	1,618	1,868	1,618	1,868
Multifamily
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	2,593	2,665	2,728	2,453
Provision for (Release of) Credit Losses	211	72	76	284,000
Financing Receivable, Allowance for Credit Loss, Ending Balance	2,804	2,737	2,804	2,737
HELOC
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	1,508	1,788	1,656	1,806
Provision for (Release of) Credit Losses	(40)	(118)	(205)	(165,000)
Recoveries	15	24	32	53
Financing Receivable, Allowance for Credit Loss, Ending Balance	1,483	1,694	1,483	1,694
Other
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance	16	313	17	353
Provision for (Release of) Credit Losses	28	(5)	46	23,000
Charge-offs	66	81	136	194
Recoveries	29	51	80	96
Financing Receivable, Allowance for Credit Loss, Ending Balance	7	278	7	278
Unfunded Loan Commitment
Allowance for loan losses:
Financing Receivable, Allowance for Credit Loss, Beginning Balance			2,700
Financing Receivable, Allowance for Credit Loss, Ending Balance	2,500	$ 2,700	2,500	$ 2,700
Unfunded Loan Commitment \| Other Liabilities
Allowance for loan losses:
ACL on unfunded commitments	$ 2,500		$ 2,500

Amount excluding accrued interest, of allowance for credit loss on financing receivable and net investment in lease.

No definition available.

Name:	osbc_FinancingReceivableAndNetInvestmentInLeaseAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable and net investment in lease from recovery.

+ References

No definition available.

+ Details

Name:	osbc_FinancingReceivableAndNetInvestmentInLeaseExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable and net investment in lease from write-off.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable and net investment in lease.

+ References

No definition available.

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482454/460-10-45-1

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnfundedLoanCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: