One full grown oscar will easily occupy your 120 gallon tank alone. I've never understood why people like the things. Biting the heater, the hand of the fishkeeper, killing any other tank inhabitants (including plecos and other armored species)...
Once they are fully mature they get this crazed buggy eyed look, if you ask me. Perhaps I am prejudiced because these fish are caught all the time in the local bodies of water that I've inhabited throughout my life (in Miami and Ocala) and are freaking enormous and seem not like aquarium fish at all, no more so than bluegills or sunfish do...so they are probably my last choice for a big tank, but I understand people have a different aesthetic than I do.
Anyhow if you have your heart set on an oscar, go for it! There are probably tons of gorgeous rescue oscars out there if you look, and an 120 gallon tank is sooo much better than a 55, which most of these guys are lucky to end up in.