Should be fine for a single oscar. You could possibly have another fish in there (like a bichir or a small group of silver dollars) but adding another cichlid is risky as he might not tolerate it.
As for the heater, if you use a sump/wetdry you can put the heater in there, otherwise you might want to shield the heater as a large oscar can surely break it. Depends on the fish though, some of them are very "mellow" and don't go around trying to break things.