A high quality pellet should be effectively their entire diet. I recommend New Life Spectrum. Oscars are particularly sensitive to HLLE which is usually caused by less than ideal food and water quality when it comes to oscars.
Live foods can introduce pathogens, are not nutritionally complete or balanced, and can increase aggression.
IMO oscars need a minimum of a 75 gallon tank for one or two, they just get too big for 55s (unless you keep them in a 55 and stunt them, in which they fish, like a kid kept in a closet always fits in the closet). But the 55 will be fine until it is about 8-10" if you do enough water changes to keep the nitrate under 20ppm.
The full size bristlenose plecos are an option as are many other like like larger Synodontis spp., silver dollars and other larger characins and cyprinids. No common plecos, they get too big, don't eat algae well, and just end up being poop machines (VERY long, stringy poop).