They eat just about EVERYTHING..to put in perspective here's what my 4 inch one eats:
Dainichi garlic cichlid pellets, hikari cichlid staple, hikari cichlid gold, frozen and freeze dried krill, frozen and live shrimp, whitefish, feeders, frozen minnows and shiners I collect down at the marsh (quarantined and frozen in almost saltwater), tetra cichlid sinking sticks, super soft ( new soft food made with krill, other type is spirulina and algae), african cichlid attack, oscar grow, prime reef flakes (by ocean nutrition), omega one algae flakes, frozen adn freeze dried bloodworms, and a heck of a lot more.
My oscar is probably on of my most healthiest fish, it's fed at least 4 different types of food everyday..
I usually go for the hikari cichlid staple and bio-gold x dainichi pellets x oscar grow diet each day