I wouldn't feed beef heart too often. Use it as a treat, let's say a couple of times a week. Your Oscars may well like it, if you feed it too often, you may find that they will turn their noses up at everything else you trying to give them. Beef heart isn't the sort of food that you should be giving to your fish on a regular basis. Give them natural foods like shrimp, worms, frozen fish. However, a good quality pellets should be the staple diet.