Xavier,
As far as I know, the name of the JDBC driver file is hard-coded in OC4J.
A "hack" (or "workaround", if you prefer :- ) would be to rename the "ojdbc14.jar" file to the file that OC4J recognizes: "classes12dms.jar", and then replace the actual "classes12dms.jar" file with your (renamed) "ojdbc14.jar" file. But I haven't done this myself, so I can't guarantee that it will work.
Why do you need the "ojdbc14.jar" JDBC driver? What does it have, that is not available in the "classes12dms.jar" driver? Perhaps the problem could be solved merely by upgrading your version of OC4J?
In case you are unaware, there are Oracle-related forums at Oracle's
OTN Web site.
Good Luck,
Avi.