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Long-Term Debt (Schedule Of Long-Term Debt Interest Rate) (Details)	12 Months Ended
	Dec. 31, 2011
Less Than .25 To 1 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.25
Greater Than Or Equal To .25 To 1 But Less Than .50 To 1 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.50
Greater Than Or Equal To .25 To 1 But Less Than .50 To 1 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.25
Greater Than Or Equal To .50 To 1 But Less Than .75 To 1 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.75
Greater Than Or Equal To .50 To 1 But Less Than .75 To 1 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.50
Greater Than Or Equal To .75 To 1 But Less Than .90 To 1 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.90
Greater Than Or Equal To .75 To 1 But Less Than .90 To 1 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.75
Greater Than .90 To 1 But Less Than Or Equal 1 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Ratio of total outstanding borrowings to borrowing Base	0.90
LIBOR [Member] | Less Than .25 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	1.50%
LIBOR [Member] | Greater Than Or Equal To .25 To 1 But Less Than .50 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	1.75%
LIBOR [Member] | Greater Than Or Equal To .50 To 1 But Less Than .75 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	2.00%
LIBOR [Member] | Greater Than Or Equal To .75 To 1 But Less Than .90 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	2.25%
LIBOR [Member] | Greater Than .90 To 1 But Less Than Or Equal 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	2.50%
ABR [Member] | Less Than .25 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	0.00%
ABR [Member] | Greater Than Or Equal To .25 To 1 But Less Than .50 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	0.25%
ABR [Member] | Greater Than Or Equal To .50 To 1 But Less Than .75 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	0.50%
ABR [Member] | Greater Than Or Equal To .75 To 1 But Less Than .90 To 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	0.75%
ABR [Member] | Greater Than .90 To 1 But Less Than Or Equal 1 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate terms, in percentage	1.00%

X

- Definition

Ratio of total outstanding borrowings to borrowing base, numerator.

+ References

No definition available.

+ Details

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Namespace Prefix:	oas_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The effective interest rate during the reporting period.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	na
Period Type:	duration