Presumably, their large bulging eyes permit good visual discrimination. Live feeder fish often bring disease into the tank, so I tend to avoid using them. Oscars are enthusiastic omnivores. But the trick is to provide them with bulky foods to satiate their enormous appetites. Obviously, flake foods are sufficient only for he smallest juveniles.
Pelleted foods are better, and for adults, floating stick foods of various sizes work well.
Another good but expensive food is freeze dried krill. Live food consisting of earthworms some insects like mealworms and crickets, snails and even crayfish work well. And yes, the occasional minnow or even goldfish will be taken with relish. Some foods to avoid, although they are relatively inexpensive, include trout chow, shrimp pellets, canned dog food, moist cat food and beef heart or beef liver. Because they are very messy foods.