Yeah that's do-able. Will have to keep on top of water changes of course when the fish are all large. And might have some aggression between the Oscars, might not, will have to give it a shot. 60 as the sump would be good, you're gonna need some good bio filtration.
And I'd agree, buy the tank before getting more fish. A lot of people cram fish into a small tank, planning to upgrade, then realize it's not so easy to find an affordable 200+ gallon setup, and the fish suffer for it.
Good luck
