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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
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Company's contractual commitments due to letters of credit |  |
Total letters of credit | $ 11,707 us-gaap_LettersOfCreditOutstandingAmount | $ 11,527 us-gaap_LettersOfCreditOutstandingAmount
Fixed |  |
Company's contractual commitments due to letters of credit |  |
Total letters of credit | 471 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember | 471 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember
Variable |  |
Company's contractual commitments due to letters of credit |  |
Total letters of credit | 11,236 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 11,056 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember
Borrower |  |
Company's contractual commitments due to letters of credit |  |
Total letters of credit | 11,131 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember | 10,955 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember
Borrower | Fixed |  |
Company's contractual commitments due to letters of credit |  |
Total letters of credit | 471 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember | 471 us-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 | 10,660 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 10,484 us-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 | 5,282 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember | 4,800 us-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 | 55 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember | 55 us-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 | 5,227 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 4,745 us-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 | 47 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember | 49 us-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 | 47 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_StandbyLettersOfCreditMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 49 us-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 | 5,802 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember | 6,106 us-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 | 416 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_FixedMember | 416 us-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 | 5,386 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditToBorrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 5,690 us-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 | 576 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember | 572 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember
Nonborrower | Variable |  |
Company's contractual commitments due to letters of credit |  |
Total letters of credit | 576 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | 572 us-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 | 576 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember | 572 us-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 | $ 576 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember | $ 572 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_FinancialInstrumentAxis = osbc_CommitmentsToExtendCreditNonborrowersMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_PerformanceGuaranteeMember / us-gaap_UnderlyingAssetClassAxis = osbc_VariableMember

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- Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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Name: | us-gaap_FinancialInstrumentAxis=osbc_CommitmentsToExtendCreditToBorrowersMember
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Data Type: | na
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- Details
Name: | us-gaap_UnderlyingAssetClassAxis=osbc_FixedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_UnderlyingAssetClassAxis=osbc_VariableMember
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