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Borrowings - Additional information (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Jan. 31, 2008
Borrowings      
Bank owned FHLBC stock 4,300,000us-gaap_FederalHomeLoanBankStock $ 4,300,000us-gaap_FederalHomeLoanBankStock  
Total borrowings 160,391,000us-gaap_DebtLongtermAndShorttermCombinedAmount 169,914,000us-gaap_DebtLongtermAndShorttermCombinedAmount  
Securities sold under repurchase agreements      
Borrowings      
Carrying amount of securities secured 26,500,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
21,000,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Fair value of the pledged collateral 43,400,000us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
43,400,000us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Number of customers having secured balances exceeding specified percentage of stockholders equity 0osbc_NumberOfCustomersHavingSecuredBalancesExceedingSpecifiedPercentageOfStockholdersEquity
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
   
Total borrowings 26,513,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
21,036,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
 
Securities sold under repurchase agreements | Minimum      
Borrowings      
Maturity 1 day    
Threshold percentage of stockholders' equity 10.00%osbc_ThresholdForDisclosurePercentageOfStockholdersEquity
/ us-gaap_DebtInstrumentAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Securities sold under repurchase agreements | Maximum      
Borrowings      
Maturity 90 days    
FHLBC advances      
Borrowings      
Borrowings at FHLBC as percentage of total assets 35.00%osbc_FederalHomeLoanBankAdvancesAsPercentageOfTotalAssets
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
Borrowings at FHLBC as percentage of book value of certain mortgage loans 60.00%osbc_FederalHomeLoanBankAdvancesAsPercentageOfBookValueOfCertainMortgageLoans
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
FHLBC advance amount 30,000,000us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
45,000,000us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
 
Bank owned FHLBC stock 4,300,000us-gaap_FederalHomeLoanBankStock
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
4,300,000us-gaap_FederalHomeLoanBankStock
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
 
Fair value of securities collateralized 74,100,000osbc_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledgedSecuritiesFairValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
Principal balance of loans collateralized 46,800,000osbc_FederalHomeLoanBankAdvancesGeneralDebtObligations
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
Combined collateral value 143,400,000us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
Amount available for additional borrowings 112,100,000us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
   
Total borrowings 30,000,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
45,000,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
 
Subordinated debt      
Borrowings      
Face amount 45,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_SubordinatedDebtMember
   
Total borrowings 45,000,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_SubordinatedDebtMember
45,000,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_SubordinatedDebtMember
 
Subordinated debt | LIBOR      
Borrowings      
Variable interest rate base three-month LIBOR    
Basis points added to reference rate (as a percent) 1.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_SubordinatedDebtMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
   
Notes payable and other borrowings      
Borrowings      
Total borrowings 500,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_NotesPayableToBanksMember
500,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_NotesPayableToBanksMember
 
Bank of America credit facility      
Borrowings      
Maximum borrowing capacity 45,500,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= osbc_CreditFacilityMember
   
Senior debt facility      
Borrowings      
Maximum borrowing capacity     30,500,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= osbc_SeniorDebtFacilityMember
Senior debt facility | LIBOR      
Borrowings      
Variable interest rate base three-month LIBOR    
Basis points added to reference rate (as a percent) 0.90%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= osbc_SeniorDebtFacilityMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
   
Senior debt facility | Prime rate      
Borrowings      
Variable interest rate base prime    
Term debt      
Borrowings      
Maximum borrowing capacity 500,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= osbc_TermDebtMember
   
Total borrowings 500,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= osbc_TermDebtMember
   
Increase in basis points on non compliance of covenant (as a percent) 2.00%osbc_DebtInstrumentIncreaseInInterestRateOnNonComplianceOfFinancialCovenant
/ us-gaap_DebtInstrumentAxis
= osbc_TermDebtMember
   
Junior subordinated debentures      
Borrowings      
Total borrowings 58,378,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_JuniorSubordinatedDebtMember
$ 58,378,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_JuniorSubordinatedDebtMember