your oscar doesn't eat pellets because it is spoiled!!!
You would have to train it to eat pellets, which takes sometimes two weeks of putting them in there and taking them out if uneaten.
It is better for the O in the long run since Os are not strict carnivores in the wild, they get fiber as well.
You can even feed Os veggies and fruits if you train them young enough.
Pellets are a good staple because the get a excellent balance of nutrients and vitamins. You don't have to worry about too much protein or fat and the health/organ problems that come with diets that are too high in those things.