I think an O ring should be just fine. You might get some bit guards just in case. How old is he? Since he hasn't been ridden in so long, I would pretty much start from square one. It's never a bad idea when you get a new horse anyway, that way you know what they have and haven't been taught and you can act accordingly. The bond you are growing is going to help a lot because he'll want to please you. I would definitely put the bit in his mouth while you work him on the ground. If you have the time leave it in his mouth for a few hours in the round pen under your watchful eye. Start very slow, just get on him and ask him to walk on . Nothing fancy, but you want him to get into the habit of moving forward instead of straight up! You should also make sure he knows what woah means, I would definitely want to make sure he has a good stop on him. Once he's relaxed w/you on him and moving forward well, ask for a little trot for a few strides and then take him back into a walk. Gradually increase what you do with him at a slow pace. Don't ask for anything faster than a trot until you are confident that he's not going to freak out.