Let me suggest that you create a separate database dedicated to RMAN and its catalog. It may be on the same machine that you have your OEM database now. After you have created your RMAN catalog, you will need to register all the DB that you want RMAN to backup with it i.e RMAN database. When you create a new database, Oracle assigns a unique DBID to the database. This unique ID is what RMAN uses to distiguish between the databases that it backs up.

A word of caution here, you cannot copy a database from another and hope RMAN will back it up because of the DBID. Also, make sure that the RMAN database is at least 816. If your rman db is 815 and all your db are 816 you may run into problem.

oisaya