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Loans - Major Classifications - Loan Concentrations (Details) (Total real estate, Loan receivables, Customer concentration risk) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Dec. 31, 2014
Total real estate | Loan receivables | Customer concentration risk |  |
Loans |  |
Loans receivable as a percentage of total portfolio | 87.90%osbc_LoansAndLeasesReceivableAsPercentageOfAggregatePortfolio / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = osbc_RealEstatePortfolioSegmentMember | 87.60%osbc_LoansAndLeasesReceivableAsPercentageOfAggregatePortfolio / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = osbc_RealEstatePortfolioSegmentMember

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- Definition Represents the percentage of total portfolio that is represented by a particular financial receivable category. + References No definition available. + Details
Name: | osbc_LoansAndLeasesReceivableAsPercentageOfAggregatePortfolio
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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