If you are going to give your oscar feeder fish, please, quarintine the feeder fish so that you don't pass on any of the feeder fish diseases to your oscars. Quarintining the feeder fish will also give you the opportunity to gut-load them. It is not the flesh of the feeder that gives your oscar its nutrition, but the stomach contents. The feeder fish must also be healthy and well-fed in order to provide nutrition to you oscar. Your fish is what it eats, so feeding your oscar starved, half-dead, sickly feeders will give you a starving, half-dead, sickly oscar.
Also, a diet of strictly feeder fish has been known to give oscars what is called hole-in-the-head disease. This disease is believed to come from a nutrional deficeny in the oscar's diet. So it is always in your best interest to feed your oscars a wide variety of foods so that they have a well-rounded nutritious diet. Feeders are usually only recommended as a once every other week or so treat, not as a staple diet. Feeding your fish a too high in protien diet can also lead to liver and kidney diseases, as well as an obeast fish.
I'd go with a cichlid staple pellet diet, and throw in feeder guppies, beefheart, worms, and other stuff as an occasional treat.
~~Colesea