Blanched lettuce works for them, I've also seen them eat zucchini, and peas. I have always had trouble getting them to leave algea wafers alone also. My Ocsars always seem to think anything I put in the tank is for them to eat. In addition to veggies, Oscars like crickets, snails, and of course gupies. I don't and never will reccomend feeder goldfish, but guppies are a very nice treat if you use good quality guppies. I'm not sure if you are using live shrimp or frozen brine, but live ghost shimp work very well. I also mix up the prepared food as they will eat almost anything, and different prepared foods have different nutritional additives.
Lazersniper makes one very good point, with the lettuce and the HITH. Oscars that recieve good nutrition and good water quality seldom have HITH IME. These fish are known to get HITH easily, but I have only seen it in poor water quality, or in a tank that gets primarily goldfish for food. I have only dealt with hith once in my life, and that was years ago with a lot less education.
Try different things You will find that your O's are pretty versatile unless the've trained you otherwise.
HTH