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Regulatory & Capital Matters - Debt and Regulatory Capital (Details) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Other regulatory capital information |  |
Subordinated debt | $ 45,000,000 us-gaap_SubordinatedDebt | $ 45,000,000 us-gaap_SubordinatedDebt
Bank of America credit facility |  |
Other regulatory capital information |  |
Subordinated debt | 45,000,000 us-gaap_SubordinatedDebt / us-gaap_DebtInstrumentAxis = osbc_CreditFacilityMember |
Debt obligation as a percent of Tier 2 regulatory capital | 60.00%osbc_DebtObligationTierTwoRegulatoryCapitalQualifyingPercentage / us-gaap_DebtInstrumentAxis = osbc_CreditFacilityMember |
Junior subordinated debentures |  |
Other regulatory capital information |  |
Trust preferred proceeds qualified as Tier 1 regulatory capital | 49,500,000 osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierOneCapital / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember | 56,600,000 osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierOneCapital / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember
Trust preferred proceeds qualified as Tier 2 regulatory capital | $ 7,100,000 osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierTwoCapital / us-gaap_DebtInstrumentAxis = us-gaap_JuniorSubordinatedDebtMember |

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- Definition Represents the percentage of debt obligation included in Tier 2 regulatory capital to the original amount of Tier 2 regulatory capital at the report date. + References No definition available. + Details
Name: | osbc_DebtObligationTierTwoRegulatoryCapitalQualifyingPercentage
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | osbc_OtherRegulatoryCapitalInformationAbstract
Namespace Prefix: | osbc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the amount of trust preferred proceeds that qualify as Tier 1 regulatory capital. + References No definition available. + Details
Name: | osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierOneCapital
Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Represents the amount of trust preferred proceeds that qualify as Tier 2 regulatory capital. + References No definition available. + Details
Name: | osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierTwoCapital
Namespace Prefix: | osbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Article 5 -Subsection 19, 20, 22 + Details
Name: | us-gaap_SubordinatedDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_DebtInstrumentAxis=osbc_CreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |