Friend of mine used to have an oscar in college. Hey, they only let you keep fish in the dorms, so this guy said "I'm gonna have a FISH!"
They get pretty enormous. I would think only 1 in that size tank, but maybe someone can correct me.
A small oscar or 2 would be OK in a 72 gallon for quite a while, but eventually they get really big. Mine was almost 14 inches before he died. When it gets that big you'll have to consider getting something even bigger, like a 150 gallon. Also, oscars are really sloppy eaters, so a good bottom feeder like a pleco is almost a must. But again, a pleco can grow pretty big in time, so eventually you'll have to upgrade to a really big tank in a year or 2.