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Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions - Commitments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Company's contractual commitments due to letters of credit
Total letters of credit	$ 11,707us-gaap_LettersOfCreditOutstandingAmount	$ 11,527us-gaap_LettersOfCreditOutstandingAmount
Fixed
Company's contractual commitments due to letters of credit
Total letters of credit	471us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember	471us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember
Variable
Company's contractual commitments due to letters of credit
Total letters of credit	11,236us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	11,056us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Borrower
Company's contractual commitments due to letters of credit
Total letters of credit	11,131us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember	10,955us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember
Borrower | Fixed
Company's contractual commitments due to letters of credit
Total letters of credit	471us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember	471us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember
Borrower | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	10,660us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	10,484us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Borrower | Financial standby
Company's contractual commitments due to letters of credit
Total letters of credit	5,282us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember	4,800us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember
Borrower | Financial standby | Fixed
Company's contractual commitments due to letters of credit
Total letters of credit	55us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember	55us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember
Borrower | Financial standby | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	5,227us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	4,745us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Borrower | Commercial standby
Company's contractual commitments due to letters of credit
Total letters of credit	47us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember	49us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember
Borrower | Commercial standby | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	47us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	49us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Borrower | Performance standby
Company's contractual commitments due to letters of credit
Total letters of credit	5,802us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember	6,106us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember
Borrower | Performance standby | Fixed
Company's contractual commitments due to letters of credit
Total letters of credit	416us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember	416us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember
Borrower | Performance standby | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	5,386us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	5,690us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Nonborrower
Company's contractual commitments due to letters of credit
Total letters of credit	576us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember	572us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember
Nonborrower | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	576us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	572us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Nonborrower | Performance standby
Company's contractual commitments due to letters of credit
Total letters of credit	576us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember	572us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember
Nonborrower | Performance standby | Variable
Company's contractual commitments due to letters of credit
Total letters of credit	$ 576us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember	$ 572us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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No definition available.

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