Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746

+ Details

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

- 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=osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
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_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=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_FiniteLivedIntangibleAssetsMember
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_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2020	Dec. 31, 2019
Assets:
Securities available-for-sale		$ 447,436	$ 484,648
Mortgage servicing rights		4,479	5,935
US Treasury Securities [Member]
Assets:
Securities available-for-sale		4,147	4,036
US Government Corporations and Agencies Securities [Member]
Assets:
Securities available-for-sale		7,276	8,337
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Assets:
Securities available-for-sale		16,779
US States and Political Subdivisions Debt Securities [Member]
Assets and liabilities measured at fair value on a recurring basis
Purchases	$ 17,554	12,800
Assets:
Securities available-for-sale		250,364	249,175
Collateralized Mortgage Obligations [Member]
Assets:
Securities available-for-sale		56,113	57,984
Asset-backed Securities [Member]
Assets:
Securities available-for-sale		80,026	81,844
Collateralized loan obligations
Assets:
Securities available-for-sale		32,731	66,684
Fair Value, Inputs, Level 1 [Member]
Assets:
Securities available-for-sale		4,147	4,036
Fair Value, Inputs, Level 2 [Member]
Assets:
Securities available-for-sale		438,611	475,193
Loans held-for-sale		9,416	3,061
Other assets		11,165	2,771
Liabilities:
Other liabilities		16,402	5,921
Level 3
Assets:
Securities available-for-sale		4,678	5,419
Mortgage servicing rights		4,479	5,935
Level 3 \| US States and Political Subdivisions Debt Securities [Member]
Assets:
Securities available-for-sale		4,700	7,200
Recurring
Assets:
Securities available-for-sale			8,337
Total financial assets		473,734	496,665
Liabilities:
Total		16,518	5,974
Recurring \| Finite-Lived Intangible Assets [Member]
Assets:
Loans held-for-sale		9,416	3,061
Mortgage servicing rights		4,479	5,935
Recurring \| Interest Rate Swap [Member]
Liabilities:
Other liabilities		16,518	5,974
Recurring \| Interest Rate Swap [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Other assets		11,165	2,771
Recurring \| Forward Contracts [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Other assets		1,238	250
Recurring \| US Treasury Securities [Member]
Assets:
Securities available-for-sale			4,036
Recurring \| US Treasury Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		4,147
Recurring \| US Government Corporations and Agencies Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		7,276
Recurring \| Mortgage Backed Securities, Issued by US Government Agencies [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		16,779	16,588
Recurring \| US States and Political Subdivisions Debt Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		250,364	249,175
Recurring \| Collateralized Mortgage Obligations [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		56,113	57,984
Recurring \| Asset-backed Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		80,026	81,844
Recurring \| Collateralized loan obligations \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		32,731	66,684
Recurring \| Fair Value, Inputs, Level 1 [Member]
Assets:
Total financial assets		4,147	4,036
Recurring \| Fair Value, Inputs, Level 1 [Member] \| US Treasury Securities [Member]
Assets:
Securities available-for-sale			4,036
Recurring \| Fair Value, Inputs, Level 1 [Member] \| US Treasury Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		4,147
Recurring \| Fair Value, Inputs, Level 2 [Member]
Assets:
Securities available-for-sale			8,337
Total financial assets		460,430	481,275
Liabilities:
Total		16,518	5,974
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Loans held-for-sale		9,416	3,061
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member]
Liabilities:
Other liabilities		16,518	5,974
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Interest Rate Swap [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Other assets		11,165	2,771
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Forward Contracts [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Other assets		1,238	250
Recurring \| Fair Value, Inputs, Level 2 [Member] \| US Government Corporations and Agencies Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		7,276
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Mortgage Backed Securities, Issued by US Government Agencies [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		16,779	16,588
Recurring \| Fair Value, Inputs, Level 2 [Member] \| US States and Political Subdivisions Debt Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		245,686	243,756
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Collateralized Mortgage Obligations [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		56,113	57,984
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Asset-backed Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		80,026	81,844
Recurring \| Fair Value, Inputs, Level 2 [Member] \| Collateralized loan obligations \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		32,731	66,684
Recurring \| Level 3
Assets:
Total financial assets		9,157	11,354
Recurring \| Level 3 \| Finite-Lived Intangible Assets [Member]
Assets:
Mortgage servicing rights		4,479	5,935
Recurring \| Level 3 \| US States and Political Subdivisions Debt Securities [Member] \| Finite-Lived Intangible Assets [Member]
Assets:
Securities available-for-sale		$ 4,678	$ 5,419