I have both a GT & JD in with my Oscar. The O is alot bigger than both and that is why I think it works. The JD & GT don'e even bother with each other but I'm sure when they get bigger it will be an issue. The O rules the tank and the JD is second. Give it a try
I have a very large oscar with a JD. They get along fine. The JD likes pots etc and the oscar is more of an open water swimmer. They rarely step on each others toes. My JD doesn't put up with any crap, but he is pretty chill.
My adult Red Oscar is the 'ruler' in the 8' tank with a 6" male GT and 3 1/2" female Severum. The Oscar was alone in the tank 1st and the other were put in later. It all depends on tank size, genders of the fish and individual personalities.