Is there a pre-defined list of printers associated with each OU ? If so, you can use either a logon script or a startup script (from GPO) that determines the ldap path for the computer. Once that is determined, you can then extract the OU info (mostly the most restricted portion of the ldap path) and compare it against the pre-defined list of OUs (and their associated printers). If one is found, then map all the printers that belong to that specific OU and delete all other printer mappings.