It will run without the O ring, but it will damage the ****** if you do not replace it. Never saw it until the next day when I cleaned the drip pan. I drove 30 miles or so, but was lucky and still haven't had any problems, that was 17k miles ago.
Very poor design by Jeep and good luck trying to find that O ring at a dealer, I couldn't find it at any of the local dealers and just reused my old one even though I thought it was slightly deformed.