Do you use the latest driver ojdbc6.jar?
Also you might set additional properties dependend to the DB driver.
e.g.:
<xa-datasource-property name="ConnectionProperties">oracle.jdbc.XATransLoose=true</xa-datasource-property>
Does the wiki help:
http://community.jboss.org/wiki/configdatasources
http://community.jboss.org/wiki/SetUpAOracleDatasource
Or you look into the docs/example/jca.