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

Amount of reduction, resulting from a sale or transfer of accretable and non-accretable loan, 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.

No definition available.

Name:	osbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAndNonAccretableDisposalsOfLoans
Namespace Prefix:	osbc_
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
Balance Type:	debit
Period Type:	instant

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 additions to the non-accretable yield the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

No definition available.

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

Amount of reduction, resulting from a sale or transfer of a non-accretable loan, 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.

No definition available.

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

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

No definition available.

Name:	osbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesNonAccretableYieldTransfersToLoansHeldForSale
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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=118938799&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=118939070&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
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of additions to 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_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAdditions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of reduction, resulting from a sale or transfer of a loan, 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=118939070&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=118938799&loc=d3e13503-111538

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldDisposalsOfLoans
Namespace Prefix:	us-gaap_
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.