Oscars are eating machines but they can become habitual in eating habits. As an example I started feeding Hikari food sticks to mine and he would tear into these like there was no tomorrow. For some reason I thought they were a bit large for some of the other fish in the tank so the next couple of times I broke the sticks up in little pellet sizes instead of the sticks. He totally ignored the food this time. He had come to recognize the shape as something he liked! I notice too that many Oscar owners tend to give food that is too large. A 10" Oscar will eat and tends to make less mess with a 3mm pellet as opposed to an 8mm. Ive also had some better feeding success with some fish using a sinking pellet vs a floating. Just dont let the fish train you too much on what it likes.