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Stockholders' Rights Plan (Details) (Rights Plan) | 3 Months Ended
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Jun. 30, 2014 individual
Rights Plan |
Rights Plan |
Number of shares issued in connection with amendment to Rights Plan | 15,525,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_ClassOfWarrantOrRightAxis = osbc_RightsPlanMember
Number of investors allowed to acquire more than 5% of common stock | 2 osbc_NumberOfInvestorsAllowedToAcquireMoreThan5PercentOfCommonStock / us-gaap_ClassOfWarrantOrRightAxis = osbc_RightsPlanMember
Percentage of common stock that may be acquired under the "Rights" Plan | 5.00%osbc_OwnershipInterestPercentageAllowedUnderPlanMinimum / us-gaap_ClassOfWarrantOrRightAxis = osbc_RightsPlanMember

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- Definition Under the terms of the Rights Plan, represents the number of investors allowed to acquire more than 5% of the entity's common stock. + References No definition available. + Details
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Namespace Prefix: | osbc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the minimum percentage of common stock that may be acquired by certain investors under terms of the Rights Plan. + References No definition available. + Details
Name: | osbc_OwnershipInterestPercentageAllowedUnderPlanMinimum
Namespace Prefix: | osbc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition Number of new stock issued during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration