hi there, are you just moving the Oscar into the 120 gallon tank, or all of the fish?if all of the fish are going into the 120 gallon tank and I don't think I would be looking for any other large Cichlid. However, if it's just the Oscar than any other fish you mention is considered perfectly okay with an Oscar.
I have never kept a Firemouth before but I know they are semi-aggressive and people often keep them with Oscars with no problems. I had a Jade eye Cichlid a couple of years ago and that made an absolutely fantastic tank mate, they don't get much bigger than 6 inches so as long as you get one that isn't going to become a potential meal, you are okay. The same goes for the fire mouth. As for a severum, these are also extremely good tank mates but do not grow very quickly. So if you do get any of these fish I would advise you try and find either a fully grown one, or at least one that is big enough to defend itself