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Loans - Inclusion (Details) (Minimum, USD $) | 3 Months Ended
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Mar. 31, 2015
Minimum |
Loans by risk rating |
Loan commitment for inclusion in credit quality analysis | $ 50,000 osbc_OutstandingLoanCommitmentsThresholdForCreditQualityCategorizing / us-gaap_RangeAxis = us-gaap_MinimumMember

X
- Definition The outstanding loan commitment to be included in credit quality indicator analysis. + References No definition available. + Details
Name: | osbc_OutstandingLoanCommitmentsThresholdForCreditQualityCategorizing
Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration