Set up an internal report parameter called @UserID (or something), and set it equal to the report global User!UserID. Make sure it's at the top of the parm list in your report, so it is determined first.
Provided you have the per-user permissions defined somewhere in your db, you can then use that parm in a query to do filtering on, in your case, your "Office Parameter". For example :
---totally made up query for Office parm dropdown ---
FROM Office AS O
INNER JOIN OfficePermissions AS OP ON O.OfficeID = OP.OfficeID
INNER JOIN Users as U ON OP.UserID = U.UserID
WHERE U.UserID = @UserID
Hope that helps!