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Stock-Based Compensation - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 3 years |  |
Number of shares available for grant | 7,200,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized |  |
Stock-based compensation expense | $ 21,302,000 us-gaap_ShareBasedCompensation | $ 11,982,000 us-gaap_ShareBasedCompensation | $ 10,333,000 us-gaap_ShareBasedCompensation
Associated tax benefit | 8,400,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 4,500,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 4,000,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Minimum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Annual Forfeiture Rate | 0.00%oas_AnnualForfeitureRate / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Annual Forfeiture Rate | 12.70%oas_AnnualForfeitureRate / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
Restricted Stock [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 3 years |  |
Stock-based compensation expense | 18,200,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 10,200,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 10,200,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Fair value of awards vested | 18,300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 6,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 11,700,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ 42.55 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 38.64 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 29.66 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Unrecognized expense | 39,600,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |
Unrecognized expense recognition period, years | 2 years 1 month 1 day |  |
Performance Share Unit Awards [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Annual Forfeiture Rate | 3.30%oas_AnnualForfeitureRate / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember |  |
Expected percentage of PSUs earned | 98.00%oas_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueOutputExpectedPercentageEarned / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | 112.00%oas_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueOutputExpectedPercentageEarned / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | 98.00%oas_SharebasedCompensationArrangementbySharebasedPaymentAwardFairValueOutputExpectedPercentageEarned / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember
Stock-based compensation expense | 3,100,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | 1,800,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | 400,000 us-gaap_ShareBasedCompensation / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ 41.71 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | $ 42.01 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember | $ 26.22 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember
Unrecognized expense | $ 9,700,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember |  |
Unrecognized expense recognition period, years | 2 years 6 months 24 days |  |
Grant date fair value amortization term | 4 years | 4 years | 4 years 0 months 2 days
Right to received shares of common stock | 1 oas_RightsReceivedByCommonShareholderForEachCommonShareOutstandingRights / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember |  |
Performance share units initial performance period, years | 3 years |  |
Number of trading days | 30 days |  |
Performance Share Unit Awards [Member] | Minimum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
PSU granted percentage of earnings | 0.00%oas_PercentageOfPerformanceShareAwardsBasedOnShareholderReturn / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
Performance Share Unit Awards [Member] | Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
PSU granted percentage of earnings | 200.00%oas_PercentageOfPerformanceShareAwardsBasedOnShareholderReturn / us-gaap_AwardTypeAxis = oas_PerformanceShareUnitAwardsMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |

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- Definition Number Of Trading Days Consider For Average Closing Price + References No definition available. + Details
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- Definition Percentage Of Performance Share Awards Based On Shareholder Return + References No definition available. + Details
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- Definition The initial performance period for the performance share units. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Output, Expected Percentage Earned + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, 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 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- 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) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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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 -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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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) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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