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Loans - Major Classifications - Loan Concentrations (Details) (Total real estate, Loan receivables, Customer concentration risk)
 3 Months Ended 12 Months Ended
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