From your description I would guess that wl is solving the "how do I use a non-xa database" problem the way jboss did in 2.x, by wrapping it in a fake xa datasource.
In jboss 3.x+ we use the local transaction jca wrapper for non-xa drivers. So, use oracle-ds.xml not oracle-xa-ds.xml.
Providing a wrapper that exposes an XAResource does not give the underlying driver xa semantics, no matter how you do it. If you need 2pc, use the oracle xa driver.