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Borrowings (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 item | Dec. 31, 2013 | Dec. 31, 2012 | Jan. 31, 2008
Borrowings |  |  |  |
Total borrowings | $ 169,914,000 us-gaap_DebtLongtermAndShorttermCombinedAmount | $ 131,438,000 us-gaap_DebtLongtermAndShorttermCombinedAmount |  |
Bank owned FHLBC stock | 4,300,000 us-gaap_FederalHomeLoanBankStock | 5,500,000 us-gaap_FederalHomeLoanBankStock |  |
Securities sold under repurchase agreements |  |  |  |
Borrowings |  |  |  |
Total borrowings | 21,036,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 22,560,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |  |
Carrying amount of securities secured | 21,000,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 22,600,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForRepurchaseAgreements / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |  |
Fair value of the pledged collateral | 43,400,000 us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 39,200,000 us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |  |
Number of customers having secured balances exceeding specified percentage of stockholders equity | 0 osbc_NumberOfCustomersHavingSecuredBalancesExceedingSpecifiedPercentageOfStockholdersEquity / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |  |  |
Additional information related to repurchase agreements |  |  |  |
Average daily balance during the year | 26,093,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 23,313,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 4,826,000 us-gaap_ShorttermDebtAverageOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
Average interest rate during the year (as a percent) | 0.01%osbc_ShortTermDebtWeightedAverageInterestRateDuringThePeriod / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.01%osbc_ShortTermDebtWeightedAverageInterestRateDuringThePeriod / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.04%osbc_ShortTermDebtWeightedAverageInterestRateDuringThePeriod / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
Maximum month-end balance during the year | 38,133,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 30,510,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 17,875,000 us-gaap_ShorttermDebtMaximumMonthendOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
Weighted average interest rate at year-end (as a percent) | 0.01%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.01%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember | 0.05%us-gaap_ShortTermDebtWeightedAverageInterestRate / us-gaap_DebtInstrumentAxis = us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember |
Securities sold under repurchase agreements | Minimum |  |  |  |
Borrowings |  |  |  |
Maturity | 1 day |  |  |
Securities sold under repurchase agreements | Maximum |  |  |  |
Borrowings |  |  |  |
Maturity | 90 days |  |  |
FHLBC advances |  |  |  |
Borrowings |  |  |  |
Total borrowings | 45,000,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember | 5,000,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |
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 |  |  |
Amount available for additional borrowings | 24,700,000 us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Rate of interest on FHLBC stock (as a percent) | 0.13%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Bank owned FHLBC stock | 4,300,000 us-gaap_FederalHomeLoanBankStock / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Fair value of securities collateralized | 57,200,000 osbc_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledgedSecuritiesFairValue / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Principal balance of loans collateralized | 47,300,000 osbc_FederalHomeLoanBankAdvancesGeneralDebtObligations / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Combined collateral value | 71,000,000 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / us-gaap_DebtInstrumentAxis = us-gaap_FederalHomeLoanBankAdvancesMember |  |  |
Subordinated debt |  |  |  |
Borrowings |  |  |  |
Total borrowings | 45,000,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_SubordinatedDebtMember | 45,000,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_SubordinatedDebtMember |  |
Face amount | 45,000,000 us-gaap_DebtInstrumentFaceAmount / 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 | 500,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_NotesPayableToBanksMember | 500,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_NotesPayableToBanksMember |  |
Bank of America credit facility |  |  |  |
Borrowings |  |  |  |
Maximum borrowing capacity | 45,500,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = osbc_CreditFacilityMember |  |  |
Number of financial covenants for which the entity continued to be out of compliance |  |  | 1 osbc_NumberOfFinancialCovenantsOutOfCompliance / us-gaap_DebtInstrumentAxis = osbc_CreditFacilityMember |
Senior debt facility |  |  |  |
Borrowings |  |  |  |
Maximum borrowing capacity |  |  |  | 30,500,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = osbc_SeniorDebtFacilityMember
Senior debt facility | LIBOR |  |  |  |
Borrowings |  |  |  |
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 |  |  |  |
Total borrowings | 500,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = osbc_TermDebtMember |  |  |
Maximum borrowing capacity | 500,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = osbc_TermDebtMember |  |  |
Basis points added to reference rate (as a percent) | 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_DebtInstrumentAxis = osbc_TermDebtMember |  |  |
Junior subordinated debentures |  |  |  |
Borrowings |  |  |  |
Total borrowings | $ 58,378,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember | $ 58,378,000 us-gaap_DebtLongtermAndShorttermCombinedAmount / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember |  |

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