Condensed Consolidating Financial Statements - Schedule of Consolidating Balance Sheet (Parenthetical) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000us-gaap_CommonStockSharesAuthorized	300,000,000us-gaap_CommonStockSharesAuthorized
Common stock, shares issued	101,627,296us-gaap_CommonStockSharesIssued	100,866,589us-gaap_CommonStockSharesIssued
Treasury stock, shares	285,677oas_TreasuryStockAtCostShares	167,155oas_TreasuryStockAtCostShares
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Common stock, par value (in usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	101,627,296	100,866,589
Treasury stock, shares	285,677	167,155

For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer.

No definition available.

+ Details

Name:	oas_TreasuryStockAtCostShares
Namespace Prefix:	oas_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Face amount or stated value per share of common stock.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

+ References

+ Details

Name:	us-gaap_CommonStockSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

+ References

+ Details

Name:	us-gaap_CommonStockSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	dei_LegalEntityAxis=us-gaap_ParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: