The OCC is an independent bureau of the Treasury Department that regulates federally chartered ("national") banks. The OCC has a useful web site (www.occ.treas.gov). Their main number is 202-874-5000, the public affairs email address is firstname.lastname@example.org, or you can look up a number or email address on the contacts page.
The OCC publishes interpretive letters, decisions, charter approvals, information about enforcement actions and other guidance in a publication called Interpretations and Actions, which the OCC posts free from May 1996.
Lexis has published and unpublished OCC interpretive letters from 1977 (BANKNG;OCCIL), enforcement decisions from 1977 (BANKNG;OCCED), Office of Financial Institution Adjudication ALJ Decisions from 1992 through 2005 (BANKNG;ALLALJ), Circulars, Bulletins & Journals from June 1968 (BANKNG;OCCBJ). There is a group file combining OCC materials (BANKNG;ALLOCC); check the Directory of Online Sources for coverage dates.
The CCH Federal Banking Law Reporter publishes selected OCC advisory letters, alerts, bulletins, staff studies, circulars, corporate decisions, memorandum, interpretations, releases, etc. The Reporter is available as a looseleaf, on Lexis (CCH;CCHFIN) and on the Intelliconnect platform (subscription only).