Name:	osbc_DividendRestrictionsAndDeferralsAbstract
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Represents the number of previous years retained profits combined with the current years profits used to determine the amount of dividends that may be paid in any calendar year.

+ References

No definition available.

+ Details

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- Definition

The period up to which the interest payments may be deferred.

+ References

No definition available.

+ Details

Name:	osbc_InterestPaymentDefermentPeriod
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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of statutory business trusts controlled by the entity that hold junior subordinated debentures.

+ References

No definition available.

+ Details

Name:	osbc_NumberOfStatutoryBusinessTrustsHoldingJuniorSubordinatedDebentures
Namespace Prefix:	osbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash outflow for accrued and unpaid interest on outstanding Trust Preferred Securities.

+ References

No definition available.

+ Details

Name:	osbc_PaymentsForAccruedAndUnpaidInterestOnOutstandingTrustPreferredSecurities
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents information pertaining to repurchase price percentage of liquidation value of stock.

+ References

No definition available.

+ Details

Name:	osbc_PreferredStockRepurchasePricePercentageOfLiquidationValue
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of dividends payable to preferred stock holders waived upon redemption of temporary equity.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the percentage of issued and outstanding stock redeemed during the period.

+ References

No definition available.

+ Details

Name:	osbc_StockRedeemedOrCalledDuringPeriodPercentageOfIssuedAndOutstandingShares
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The minimum percentage of Tier 1 Common Capital required to be maintained in the next fiscal year following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageNextTwelveMonths
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Balance Type:	na
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- Definition

The minimum percentage of Tier 1 Common Capital required to be maintained in year four and thereafter following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

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- Definition

The minimum percentage of Tier 1 Common Capital required to be maintained in year three following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The minimum percentage of Tier 1 Common Capital required to be maintained in year two following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearTwo
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Data Type:	num:percentItemType
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- Definition

Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

+ 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 S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 405

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

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Balance Type:	debit
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- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ 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

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow to reacquire preferred stock during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash outflow in the form of ordinary dividends to preferred shareholders of the parent entity.

+ References

+ Details

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- Definition

The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph d

-Article 4

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(d)(1))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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- Definition

Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.28)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29

-Article 5

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Balance Type:	na
Period Type:	instant

- Definition

Number of stock bought back by the entity at the exercise price or redemption price.

+ References

Reference 1: 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

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Namespace Prefix:	us-gaap_
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Period Type:	duration

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Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details) (USD $)	0 Months Ended					1 Months Ended	3 Months Ended
	Feb. 15, 2015	Dec. 30, 2014	May 15, 2014	Apr. 28, 2014	Apr. 15, 2014	May 31, 2014	Mar. 31, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 30, 2014	Apr. 28, 2014
Dividend Restrictions and Deferrals
Number of previous years retained profit considered for dividend payment							2 years
Tier 1 Common Equity, minimum percentage required, 2016							0.625%
Tier 1 Common Equity, minimum percentage required, 2017							1.25%
Tier 1 Common Equity, minimum percentage required, 2018							1.875%
Tier 1 Common Equity, minimum percentage required, 2019 and thereafter							2.50%
Junior subordinated debenture							58,378,000		58,378,000
Payments to indentured trustees								19,700,000
Dividends paid							1,006,000
Preferred stock, Liquidation value (in dollars per share)							1,000		1,000
Preferred Stock, Liquidation Preference Per Share							1,000		1,000
Preferred stock, Shares outstanding							31,553us-gaap_PreferredStockSharesOutstanding		47,331us-gaap_PreferredStockSharesOutstanding
Series B Stock
Dividend Restrictions and Deferrals
Preferred stock dividend amount					15,800,000
Dividends paid	710,000		10,300,000			10,300,000
Shares redeemed (in shares)				25,669			15,778	25,669
Repurchase price percentage of liquidation value (as a percent)				94.75%				94.75%
Preferred stock, Liquidation value (in dollars per share)				$ 947.50							$ 947.50
Preferred Stock, Liquidation Preference Per Share				$ 947.50							$ 947.50
Payments for repurchase of preferred stock				24,300,000				24,300,000
Benefit from reversal of dividends previously accrued or accumulated and waived by holders upon redemption								5,400,000
Preferred stock, Shares outstanding		47,331					31,553		47,331	47,331
Shares to be redeemed, percent of issued and outstanding		33.33%					33.33%
Series B Stock \| Large private investor
Dividend Restrictions and Deferrals
Payments for repurchase of preferred stock				22,900,000
Series B Stock \| Directors
Dividend Restrictions and Deferrals
Payments for repurchase of preferred stock				1,400,000
Junior subordinated debentures
Dividend Restrictions and Deferrals
Junior subordinated debenture							58,400,000
Number of statutory business trusts that hold junior subordinated debentures							2
Junior subordinated debentures \| Maximum
Dividend Restrictions and Deferrals
Number of quarterly periods for deferral of interest							20