oscar food
Oscars are carnivors so anything meaty is apprecited. Earthworms are excellent but I'd advise squeezing out all the poop first. If you don't want to get this messy you can put them in a tupperware container with just moist papertowel for a few days until all the poop is excreated. Probably the best worms to use would be red worms, the kind used for indoor composters-essentially culturing a free food source. Additional good foods would be beefheart and I have yet to meet a fish that didn't like mytsis shrimp.
Also spreading the feedings out over the course of the day may leave less waste, oscars are messy by nature.