Firstly you can look at your lighting, a high intensity lamp with peak emissions in the blue and red regions to maximize the photobiological processes in plants is often great at bringing out the colour of your fish, especially the reds. I recommend Zoomed florasun.
If you then wanted to look at colour enhancing foods, Sera do a great one enriched with South Antarctic Krill, Algae Meal, Spirulina, and Beta Carotene. These will enhance the 'red' in your fish. Never use anything artificial as this could ultimately damage your Oscar.
Happy Fishkeeping!