Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 506,877us-gaap_NetIncomeLoss	$ 227,959us-gaap_NetIncomeLoss	$ 153,388us-gaap_NetIncomeLoss
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	412,334	307,055	206,734
Gain on sale of properties	(186,999)us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty
Impairment of oil and gas properties	47,238us-gaap_ImpairmentOfOilAndGasProperties	1,168us-gaap_ImpairmentOfOilAndGasProperties	3,581us-gaap_ImpairmentOfOilAndGasProperties
Deferred income taxes	307,457us-gaap_DeferredIncomeTaxExpenseBenefit	134,583us-gaap_DeferredIncomeTaxExpenseBenefit	92,479us-gaap_DeferredIncomeTaxExpenseBenefit
Derivative instruments	(327,011)	35,432	(34,164)
Stock-based compensation expenses	21,302us-gaap_ShareBasedCompensation	11,982us-gaap_ShareBasedCompensation	10,333us-gaap_ShareBasedCompensation
Deferred financing costs amortization and other	11,028oas_DebtDiscountAmortizationAndOther	4,248oas_DebtDiscountAmortizationAndOther	2,810oas_DebtDiscountAmortizationAndOther
Working capital and other changes:
Change in accounts receivable	16,702	(107,473)	(90,103)
Change in inventory	(3,776)us-gaap_IncreaseDecreaseInInventories	(13,941)us-gaap_IncreaseDecreaseInInventories	(29,313)us-gaap_IncreaseDecreaseInInventories
Change in prepaid expenses	(3,199)	(8,191)	346
Change in other current assets	(6,135)	(56)	156
Change in other assets	114	(3,248)	(95)
Change in accounts payable and accrued liabilities	76,723	107,451	76,706
Change in other current liabilities	0	0	(472)
Change in other liabilities	(139)	887	0
Net cash provided by operating activities	872,516	697,856	392,386
Cash flows from investing activities:
Capital expenditures	(1,354,281)	(893,524)	(1,051,365)
Acquisition of oil and gas properties	(46,247)	(1,560,072)	0
Proceeds from sale of properties	324,852	0	0
Costs related to sale of properties	(2,337)	0	0
Derivative settlements	6,774	(8,133)	6,545
Purchases of short-term investments	0	0	(126,213)
Redemptions of short-term investments	0	25,000	120,316
Advances from joint interest partners	(6,213)	(8,347)	12,112
Net cash used in investing activities	(1,077,452)	(2,445,076)	(1,038,605)
Cash flows from financing activities:
Proceeds from issuance of senior notes	0	1,000,000	400,000
Proceeds from revolving credit facility	620,000	600,000	0
Principal payments on revolving credit facility	(455,570)	(264,430)	0
Debt issuance costs	(99)us-gaap_PaymentsOfDebtIssuanceCosts	(22,910)us-gaap_PaymentsOfDebtIssuanceCosts	(8,012)us-gaap_PaymentsOfDebtIssuanceCosts
Proceeds from sale of common stock	0	314,580	0
Purchases of treasury stock	(5,309)	(1,566)	(3,194)
Other	(176)	0	0
Net cash provided by financing activities	158,846	1,625,674	388,794
Decrease in cash and cash equivalents	(46,090)	(121,546)	(257,425)
Cash and cash equivalents:
Beginning of period	91,901	213,447	470,872
End of period	45,811	91,901	213,447
Supplemental cash flow information:
Cash paid for interest, net of capitalized interest	150,181us-gaap_InterestPaidNet	85,596us-gaap_InterestPaidNet	53,488us-gaap_InterestPaidNet
Cash paid for taxes	5,329us-gaap_IncomeTaxesPaid	750us-gaap_IncomeTaxesPaid	107us-gaap_IncomeTaxesPaid
Supplemental non-cash transactions:
Change in accrued capital expenditures	169,710	34,354	59,878
Change in asset retirement obligations	$ 6,182oas_NonCashAssetRetirementObligations	$ 13,201oas_NonCashAssetRetirementObligations	$ 10,230oas_NonCashAssetRetirementObligations

The total of the component of interest expense representing the noncash expenses charged against earnings in the period to amortize the long-term deferred financing costs and other income (expense) included in net income that results in no cash inflows or outflows in the period. Includes noncash adjustments to reconcile net income (loss) to cash provided by (used in) operating activities that are not separately disclosed.

No definition available.