MSAccess has a limit of 255 columns on any queries that are run locally with the MSJet engine, and the 255 limit also impacts the list of fields that MSAccess pulls from the database and presents to you in the drop-down menu of field names (i.e., Access only shows you the first 255 fields).

In the old Oracle9i driver, I could code a "pass-thru" SQL query that would send the SQL string directly to the Oracle server and run it at the server (instead of running it locally in MSJet engine).  I could include any fieldname in this pass-thru query string and Oracle would return the data to me, thereby working-around the 255 field limit in Access (plus the server-side queries are much faster since Oracle does all the lifting server-side and just returns the results via XML stream to the client)