Loans and Allowance for Credit Losses on Loans - Aging Analysis (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Aged analysis of past due loans
Non accrual including PCI Loans	$ 35,973	$ 41,531
Total Loans	3,402,370	3,421,948
Total loans, including deferred loan loan costs and PCI	3,358,062	3,377,667
Recorded Investment 90 days or Greater Past Due and Accruing	743
Total Loans		3,110
Special Mention [Member]
Aged analysis of past due loans
Total Loans	113,708	48,236
Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	38,317
Total Past Due Including PCI loans		27,342
30 to 59 Days Past Due
Aged analysis of past due loans
Current	21,079
Total Past Due Including PCI loans		9,342
60 to 89 Days Past Due
Aged analysis of past due loans
Current	839
Total Past Due Including PCI loans		4,356
90 Days or Greater Past Due
Aged analysis of past due loans
Current	16,399
Total Past Due Including PCI loans		13,644
Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	3,364,053
Current including PCI Loans		3,394,606
Commercial
Aged analysis of past due loans
Nonaccrual	9,029	11,894
Total Loans	695,545	771,474
Recorded Investment 90 days or Greater Past Due and Accruing		1,396
Commercial \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	4,971	4,530
Commercial \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	12,576	6,648
Commercial \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	4,904	3,407
Commercial \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current	548	1,413
Commercial \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	7,124	1,828
Commercial \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	682,969	764,826
Leases
Aged analysis of past due loans
Nonaccrual	2,641	3,754
Total Loans	211,132	176,031
Leases \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	303
Leases \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	1,937	1,696
Leases \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	401	125
Leases \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	1,536	1,571
Leases \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	209,195	174,335
Commercial real estate - Investor
Aged analysis of past due loans
Nonaccrual	6,335	5,694
Total Loans	965,767	957,389
Commercial real estate - Investor \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	66,015	26,016
Commercial real estate - Investor \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	3,400	1,374
Commercial real estate - Investor \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	3,399
Commercial real estate - Investor \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current		267
Commercial real estate - Investor \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	1	1,107
Commercial real estate - Investor \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	962,367	956,015
Commercial real estate - Owner occupied
Aged analysis of past due loans
Nonaccrual	9,871	11,637
Total Loans	655,792	574,384
Recorded Investment 90 days or Greater Past Due and Accruing		1,594
Commercial real estate - Owner occupied \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	18,277	2,953
Commercial real estate - Owner occupied \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	12,820	7,672
Commercial real estate - Owner occupied \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	9,764	2,324
Commercial real estate - Owner occupied \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current	100	500
Commercial real estate - Owner occupied \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	2,956	4,848
Commercial real estate - Owner occupied \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	642,972	566,712
Real estate - construction
Aged analysis of past due loans
Nonaccrual	155	160
Total Loans	165,558	206,132
Recorded Investment 90 days or Greater Past Due and Accruing	7
Real estate - construction \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	124	854
Real estate - construction \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	117	854
Real estate - construction \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	7
Real estate - construction \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	165,434	205,278
Residential real estate - Investor
Aged analysis of past due loans
Nonaccrual	1,391	876
Total Loans	62,846	63,399
Recorded Investment 90 days or Greater Past Due and Accruing	2	23
Residential real estate - Investor \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	1,270	1,657
Residential real estate - Investor \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	269	395
Residential real estate - Investor \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current		470
Residential real estate - Investor \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	1,001	792
Residential real estate - Investor \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	61,576	61,742
Residential real estate - Owner occupied
Aged analysis of past due loans
Nonaccrual	4,321	4,898
Total Loans	203,118	213,248
Recorded Investment 90 days or Greater Past Due and Accruing	20	97
Residential real estate - Owner occupied \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	638	659
Residential real estate - Owner occupied \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	4,492	5,662
Residential real estate - Owner occupied \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	1,805	1,994
Residential real estate - Owner occupied \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current	98	591
Residential real estate - Owner occupied \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	2,589	3,077
Residential real estate - Owner occupied \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	198,626	207,586
Multifamily
Aged analysis of past due loans
Nonaccrual	938	1,573
Total Loans	298,686	309,164
Recorded Investment 90 days or Greater Past Due and Accruing	691
Multifamily \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	6,891	6,900
Multifamily \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	691	1,046
Multifamily \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current		1,046
Multifamily \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	691
Multifamily \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	297,995	308,118
HELOC
Aged analysis of past due loans
Nonaccrual	1,116	862
Total Loans	110,688	115,664
Recorded Investment 90 days or Greater Past Due and Accruing	23
HELOC \| Special Mention [Member]
Aged analysis of past due loans
Total Loans	108	108
HELOC \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	766	434
HELOC \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	354	193
HELOC \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current	93	23
HELOC \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	319	218
HELOC \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	109,922	115,230
HELOC - Purchased
Aged analysis of past due loans
Nonaccrual	173	180
Total Loans	9,553	10,626
HELOC - Purchased \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	173	180
HELOC - Purchased \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	173	180
HELOC - Purchased \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	9,380	10,446
Other
Aged analysis of past due loans
Nonaccrual	3	3
Total Loans	23,685	24,437
Other \| Financial Asset, Past Due [Member]
Aged analysis of past due loans
Current	68	119
Other \| 30 to 59 Days Past Due
Aged analysis of past due loans
Current	66	50
Other \| 60 to 89 Days Past Due
Aged analysis of past due loans
Current		46
Other \| 90 Days or Greater Past Due
Aged analysis of past due loans
Current	2	23
Other \| Financial Asset, Not Past Due [Member]
Aged analysis of past due loans
Current	$ 23,617	$ 24,318

Amount of financing receivable on nonaccrual status including purchased credit impaired loans.

No definition available.

Name:	osbc_FinancingReceivableNonaccrualIncludingPurchasedCreditImpairedLoans
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable not past due including purchased credit impaired loans.

+ References

No definition available.

+ Details

Name:	osbc_FinancingReceivableNotPastDueIncludingPurchasedCreditImpairedLoans
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable past due including purchased credit impaired loans.

+ References

No definition available.

+ Details

Name:	osbc_FinancingReceivablePastDueIncludingPurchasedCreditImpairedLoans
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans and purchased credit impaired loans. Excludes loans and leases covered under loss sharing agreements.

+ References

No definition available.

+ Details

Name:	osbc_LoansAndLeasesReceivableIncludingPurchasedCreditImpairedLoansNetOfDeferredIncome
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922355-210448

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5111-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919258-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (2)

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

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 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953401-111524

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919253-210447

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922355-210448

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (2)

Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1404
-Paragraph (a)

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancialAssetNotPastDueMember
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:

- 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=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=osbc_HelocPurchasedMember
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: