I get frozen beefheart gumdrops for mine...he loves them so much, I have trouble getting him to eat his pellets...
feeding red meats and other similar foods that they would not normally eat is not recommended, as the fat and nutrient content is horribly unbalanced from their true nutritional needs.
as for feeding live foods (rosy reds, goldfish, guppies, minnows, whatever you want to call them), that is not a good practice either. once again, nutrition prevails. remember what happened to the guy who ate McDonald's for a month straight? There is not much difference in the nutrition of fast food to us compared to live goldfish/feeders to oscars. Pellets, sticks, and flakes were formulated specially for oscars with their proper nutrition in mind.
If you want your fish to get big quick and be susceptible to disease, illness, and live a shorter life, feed it live foods exclusively. if you want your oscar to grow at a normal rate and be more colorful, lively, and healthy, feed it primarily pelleted foods, with the occasional live feeder (not more than twice a month IMO).
everyone assumes that since oscars are such voracious predators when it comes to small fish, that that is their normal diet. such is not the case.