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Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Income tax benefit	$ 7,376,000us-gaap_IncomeTaxExpenseBenefit	$ (101,519,000)us-gaap_IncomeTaxExpenseBenefit
Effective tax rate	29.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations	37.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Income Tax Benefit, Excluding Discrete Tax Items	$ 10,600,000oas_IncomeTaxBenefitExcludingDiscreteTaxItems
Income Tax Benefit, Excluding Discrete Tax Items, Percentage of Loss Before Tax	41.60%oas_IncomeTaxBenefitExcludingDiscreteTaxItemsPercentageofLossBeforeTax

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Income Tax Benefit, Excluding Discrete Tax Items

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Income Tax Benefit, Excluding Discrete Tax Items, Percentage of Loss Before Tax

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.4-08.(h)(2))

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Reference 1: 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.(h))

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-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6515339



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