www.aquariumfishworld.com/2008/12/what-to-feed-oscar-fish/
Food & Care:
Remember that your Oscar is primarily carnivorous (eats meat), so provide him with a high protein diet. Many high quality processed fish foods are available on the market today, and most frozen fish foods are also appropriate. In the wild, most Oscars eat primarily small fish, aquatic invertebrates, insects, and insect larvae, only opportunistically scavenging on an old carcass, nipping at the fins or scales of larger fish, or consuming plant matter, so you are best off to try to duplicate this in the aquarium environment. Provide a wide variety of processed, frozen, and freeze dried foods. Your fish should be able to consume all the food you offer within two minutes of feeding. If there is food in the tank after this time has elapsed, this contributes to poor water quality and make your Oscars more susceptible to disease. Though Oscars will take live foods, feeding live goldfish is always a bad idea. Feeder goldfish are not a very nutritious food, and they are a good way to spread disease to your beloved Oscars. You only need to feed your Oscar once a day. If you feed more than once a day you increase the waste your fish are producing and also increase the risk of over feeding, so it becomes more important that you are performing your weekly 10-15% water changes and all necessary filter maintenance. Providing a proper diet increases your Oscar's life expectancy and bolsters their immune system.
Oscars are less aware than most fish that they are prey animals. However, in order to feel comfortable, they need to have lots of hiding places. Oscars like to rearrange their tanks, so, unless you don't mind the Oscar moving everything around on you, stick with large, heavy pieces that will be harder for the Oscar to get a hold of and move. Remember, you should avoid using sharp or rough decorations that your fish may get hurt on, in case your fish dart into them when startled. Like most other fish, they are more comfortable and will come out more and be more active if you can provide them with 50-75% cover - this means you should make half to three fourths of your tank space hiding places. Providing sufficient cover reduces stress and therefore improves your Oscar's immune system, reducing chance of disease.