In terms of veggies, if it looks like a pellet (i.e. peas) they'll go for it at any size. If its just a piece/slice of a vegetable then you have to train them to eat it at a young age.
They do need meat but Oscars ARE OMNIVORES. They need greens to keep their intestinal tract functioning correctly and a supply of plant nutrients. You do not want bloat or constipation with a fish that loves eating.
my oscar also loves spirulina crumbles-there is a man on ebay who sells it. along with krill, brine shrimp, blood worms-im going to try earthworms next.