A regular dee ring is good. I personally like the way eggbutts look better, especially if the horse has a small mouth. I like using copper in my bits. It makes the horse's mouth stay nice and wet aka it encourages saliva. These are ones I like:
I liked the look too. Plus it's german silver/copper enhanced, so should(?) have some taste in it. :) That's the 2nd bit I got for my youngsters. My paint doesn't like sidepull/hackamore, so I had no choice bu find a mild bit. They both didn't like Korsteel french eggbutt, but seem to be fine with this one. So far I have no complaints.
On all of my well broke horses (all western), I use a very short shank medium ported curb bit with swivel shanks. It has the potential to be a severe bit in severe hands but with a soft touch works great with neck reining horses.
Personally I use an Eggbutt French Link. In my opinion it is one of the softest bits you can get - it doesn't have the crackerjack action or single hinge action of a snaffle, and you can't pinch a horse's mouth with an Eggbutt piece.
Harlee, forgive me, but it would be really helpful if you explained what exactly you meant (o-ring can be any type of mouthpiece, just means that the rings are loose ring.... could range from a snaffle to a double twisted wire) and why you would recommend it. There might be other readers that are intersted.
Just a suggestion; three-word responses are very vague ;)