tank raised oscars generally only get about a foot long, wild oscars have been seen at 16" or even 18", a 12" fish is ok in a 50 gallon wide, not great but ok, better in a 75 but not great either
does a fish need a tank 3 or 4 times it's body length, that's personal opinion but we both agree that 4 times is better, i just think 3 times is ok too, of course both are cruel but that's the tragedy of keeping fishes in tanks in our homes