No definition available.

Name:	osbc_DividendRestrictionsAndDeferralsAbstract
Namespace Prefix:	osbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

Name:	osbc_DividendRestrictionsNumberOfPreviousYearsRetainedProfit
Namespace Prefix:	osbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	osbc_InterestPaymentDefermentPeriod
Namespace Prefix:	osbc_
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

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
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 year four and thereafter following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearFourAndThereafter
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 year three following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

Name:	osbc_TierOneCommonEquityCapitalDistributionLimitationMinimumRegulatoryCapitalPercentageYearThree
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 year two following the latest fiscal year to prevent capital distribution limitations under regulatory capital rules.

+ References

No definition available.

+ Details

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

- 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 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 2: 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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

+ Details

Name:	us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
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Regulatory & Capital Matters - Dividend Restrictions and Deferrals (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Dividend Restrictions and Deferrals
Number of previous years retained profit considered for dividend payment		2 years
Minimum capital requirements, minimum percentage required, 2016		0.625%
Mimimum capital requirements, minimum percentage required, 2017		1.25%
Mimimum capital requirements, minimum percentage required, 2018		1.875%
Mimimum capital requirements, minimum percentage required, 2019 and thereafter		2.50%
Junior subordinated debenture		$ 58,378	$ 58,378
Payments to indentured trustees	$ 19,700
Junior subordinated debentures
Dividend Restrictions and Deferrals
Junior subordinated debenture		$ 58,400
Number of statutory business trusts that hold junior subordinated debentures \| item		2
Junior subordinated debentures \| Maximum
Dividend Restrictions and Deferrals
Number of quarterly periods for deferral of interest \| item		20