If you have just defeated the Overdrive Sin, you are not quite to the end. You still have one more boss to fight, and then the final sequence with a boss, and two more battles that really only serve to wrap up the plot.
You do not *need* to have any of the Celestial Weapons finished, but it will make the final battles much easier if you have one or two characters that can hit for more than 9999dmg. It is definitely not necessary; using lots of overdrives and the usual battle tactics should suffice.
If you have any other questions ask away, and I can try to help with battle strategy if you get there and have trouble. For now I have left my info spoiler free, since you haven't finished. Good Luck.