The DefaultDS will be used for EJB2 CMP (Entity) persistence, JEE timers and JBossMessaging.
So after starting Server you should have a table TIMERS and several JMS tables.
You might change the persistence plugin for deploy/ejb2-timer-service.xml from GeneralPurpose to Oracle (example within the file)
If you have a example for timers and for JMS you might try this, otherwise I would say it will be ok if you don't see errors during startup :)