I am a massage therapist who specializes in neuromuscular therapy. I have two clients that are also using hyperbaric oxygen therapy, is this ok?

Quite ok. No problem of massage therapy in patient using hyperbaric oxygen.
Not sure. I am not sure what neuromuscular therapy is (massage therapy? Physical training? A form of rehab?). I imagine that your client is getting neuromuscular therapy, and getting hyperbaric oxygen therapy (hbot) for other reasons (i am not aware of any uses of hyperbaric oxygen for neuromuscular-type therapies). I think that this is probably ok, as hbot has few (if any) muscle side effects.
No !! The application of hyperbaric oxygen is limited, such as severe peripheral vascular disease and danger of gangrene or in carbon monoxide poisoning , I am not aware of any benefit musle related problems!