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Stock-Based Compensation - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 7,606us-gaap_ShareBasedCompensation	$ 4,505us-gaap_ShareBasedCompensation
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Restricted stock granted	1,219,820us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Granted, weighted average grant date fair value per unit	$ 13.09us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Stock-based compensation expense	6,800us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	3,900us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Restricted Stock [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual forfeiture rate	0.00%oas_AnnualForfeitureRate / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Restricted Stock [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual forfeiture rate	16.90%oas_AnnualForfeitureRate / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted	425,590us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Granted, weighted average grant date fair value per unit	$ 11.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Stock-based compensation expense	$ 800us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember	$ 600us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Right to received shares of common stock	1oas_RightsReceivedForEachPerformanceShareUnit1 / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Performance share units initial performance period, years	3 years
Grant date fair value amortization term	4 years
Number of trading days	30 days
Performance Share Units [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual forfeiture rate	2.40%oas_AnnualForfeitureRate / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
PSU granted percentage of earnings	0.00%oas_PercentageOfPerformanceShareAwardsBasedOnShareholderReturn / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Performance Share Units [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual forfeiture rate	4.90%oas_AnnualForfeitureRate / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
PSU granted percentage of earnings	200.00%oas_PercentageOfPerformanceShareAwardsBasedOnShareholderReturn / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember
PSU 2015 Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value amortization term	4 years
Expected percentage earned	86.00%oas_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueOutputExpectedPercentageEarned / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitsTwoThousandFourteenMember

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- Definition

The rate of forfeitures assumptions that is used in calculating the fair values of a share-based compensation award.

+ References

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- Definition

Number Of Trading Days Consider For Average Closing Price

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- Definition

Percentage Of Performance Share Awards Based On Shareholder Return

+ References

No definition available.

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- Definition

Rights Received For Each Performance Share Unit1

+ References

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X

- Definition

The initial performance period for the performance share units.

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Output, Expected Percentage Earned

+ References

No definition available.

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X

- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

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-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

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