Name:	osbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableAndNonAccretableYield
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Amount of reduction, resulting from a reclassification to held-for-sale, in the accretable and non-accretable yield on acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

Name:	osbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableAndNonAccretableYieldTransfersToLoansHeldForSale
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of non-accretable discount on acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

Name:	osbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesNonAccretableYield
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
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Amount of accretion recognized for non-accretable yield on loans acquired loans not accounted for as debt securities, with evidence of deterioration of credit quality.

No definition available.

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

Amount of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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

Amount of accretion recognized for the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities, with evidence of deterioration of credit quality.

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAccretion
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

No definition available.

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

Amount of reduction, resulting from a reclassification to held-for-sale, in the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538