What version of Oracle are you using? Version 10g should be unsing an undo segment in an undo tablspace and you don't need to do anything else. For older version you should create a rollback tablesapce and assign public (or private) rollbacksegments in that tablespace that are available to be used by all users of the DB. The difference between public and private is that private rollbacks must be stated by being placed in the init.ora file and are avabilable to that instance only. A public rollback is stated when an instacne that uses that rollback segment is started and can be shared by more then one instance.