From Secrecy News:
Legal opinions issued by the Justice Department Office of Legal Counsel that interpret the law for the executive branch on questions of surveillance, detention and other disputed national security policies are among the Bush Administration records that are most urgently sought by members of Congress and others, and are often among the records that are most tightly withheld.
http://www.fas.org/blog/secrecy/2008/12/access_to_olc.html