and now I need to make it run on OC4J 10g the current production version only support J2EE 1.3 (actually information on the oracle website is contradictory) which only support (if I'm right) JSP 1.2 and Servlet 2.3
Now the question is : How would you tackle this? What are the main issues to look for?
Start by trying to compile everything against 1.3. That should give you an idea of what you're using that's not supported, and how much work it might be to correct. As for the JSP, just throw it on the server and see what happens. The developers should also be able to give you some clues what features they used that might be new.