If you are writing a thesis or dissertation for the Oxford Law Faculty, follow US practice for the citation of primary sources (legislation, cases) - then apply OSCOLA's principles for consistency & clarity. (OSCOLA (4th edn, 2012) paras 2.8.1, 2.8.2)
"Under the Judiciary Act of 1789, the Attorney General was authorized to render opinions on questions of law when requested by the President and the heads of Executive Branch departments. This authority is now codified at 28 U.S.C. §§ 511-513. Pursuant to 28 U.S.C. § 510, the Attorney General has delegated to the Office of Legal Counsel responsibility for preparing the formal opinions of the Attorney General, rendering opinions and legal advice to the various Executive Branch agencies, assisting the Attorney General in the performance of his function as legal adviser to the President, and rendering opinions to the Attorney General and the heads of the various organizational units of the Department of Justice. 28 C.F.R. § 0.25." (USDOJ website, viewed 23 November 2012 http://www.justice.gov/olc/opinions.htm)
Official Opinions of the Attorneys General of the United States
|USA 120 A30 (sec coll)|
You may find these cited as Op. Att'y Gen.
Holders of an Oxford SSO can access U.S. Attorney General Opinions (from 1791 on) via the Westlaw US subscription