I feed my oscar pellots and he gets goldfish once a week or so. I also give him an earthworm every other day or so. The lady at the pet store said I should supplement that with beef heart. She said that is a good staple that has the nutrients he needs. Unfortunately, none of my local grocery stores have beef heart. Would liver work? If not, what would be a good staple for him? Apparently there is not alot of nutrition in gold fish.

Their main food should be a good meat-based cichlid pellet. Maybe something like this:

http://www.drsfostersmith.com/Product/Prod...362&N=2004+6105

Beef heart is high in things like protein and iron. They sell it packed in the frozen fish section. Here's the stuff I mean:
http://www.drsfostersmith.com/Product/Prod...=1&N=2004&Nty=1

You may be able to get plain beef heart from a traditional butcher shop or an Oriental market.

In addition to live fish and earthworms, you can try crickets and mealworms (one at a time, they drown pretty soon if not eaten).

Oscars don't need live fish. They can introduce parasites, bacteria, and/or fungus since most feeder fish are poorly treated. If you really want to use live fish, you can vary it from goldfish to rosy red minnows to guppies; those are the "feeders" they sell around here. Smaller oscars enjoy bloodworms and other smaller insect larvae and worms too.