The old ball joint that came with the moog lower control arm assembly has a snap ring on it. I am replacing it with a new one from Oreilly's that does not come with a snap ring? My question is: should I reuse the snap ring or should I just forgo it completely with the new part?
You'll want to remove the snap ring, then remove the old ball joint, replace the ball joint, and then replace the snap ring. If it's damaged, you should replace the snap ring.
Do NOT replace the ball joint without re-installing the snap ring as this type of ball joint relies on the snap ring to hold things in place.