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

No definition available.

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

- 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

- Definition

Value of input used to measure servicing asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_ServicingAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=osbc_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputConstantPrepaymentRateMember
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:

Fair Value Measurements - Quantitative and Qualitative Information (Details) $ in Thousands	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	$ 5,361	$ 7,357
Securities available-for-sale, at fair value	488,422	541,248	$ 488,422
Fair Value, Inputs, Level 3 [Member]
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	5,361	7,357
Securities available-for-sale, at fair value	5,873	8,165		$ 9,500
US States and Political Subdivisions Debt Securities [Member]
Quantitative information about Level 3 fair value measurements
Securities available-for-sale, at fair value	238,727	274,067
US States and Political Subdivisions Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Quantitative information about Level 3 fair value measurements
Securities available-for-sale, at fair value	5,900
Collateralized Mortgage Obligations [Member]
Quantitative information about Level 3 fair value measurements
Securities available-for-sale, at fair value	$ 64,121	$ 64,429
Collateralized Mortgage Obligations [Member] \| Fair Value, Inputs, Level 3 [Member]
Quantitative information about Level 3 fair value measurements
Securities available-for-sale, at fair value				$ 1,600
Measurement Input, Discount Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input		10.2
Measurement Input, Discount Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Minimum [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input		10.0
Measurement Input, Discount Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Maximum [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input		229.7
Measurement Input, Discount Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Weighted Average [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input	10.1
Measurement Input, Constant Prepayment Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input		9.6
Measurement Input, Constant Prepayment Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Minimum [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input	7.0	7.0
Measurement Input, Constant Prepayment Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Maximum [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input	69.0	68.9
Measurement Input, Constant Prepayment Rate \| Mortgage Servicing Rights [Member] \| Fair Value, Inputs, Level 3 [Member] \| Weighted Average [Member]
Quantitative information about Level 3 fair value measurements
Servicing Asset, Measurement Input	16.6
Fair Value, Measurements, Recurring [Member]
Quantitative information about Level 3 fair value measurements
Securities available-for-sale, at fair value		$ 10,951