I would put both Oscars in and watch them. Putting both of them insures that it wouldn’t be just one new fish put in for every fish to test, and also the 2 Oscar can give each other support.
You could probably get away with adding him because of size and temperament, but always need a back up plan. I find most time Oscars just ignore other fish and so their own thing
I have restored several cars from the 1960s and 1970s and I know how much work it requires. In my opinion, leave the Oscars in the second tank. New fish will arrive and there will be chaos here.
