well stay away from those feeder goldfish, but other than that you can pretty much feed him anything thats small enough for him to swallow lol, he should definitely be on a staple diet of a good pellet thats small enough for him, but as for treats you can do worms (just cut up into pieces since he is small) or small crickets/grasshoppers, krill, shrimp etc. just make sure these are only treats and hes getting a balanced pellet as his main food, also keep in mind if this is your first oscar, that he is ALWAYS going to look like hes searching for food and starving to death, its just how they are and part of their charm lol, its very easy to overfeed these guys because they are always begging for food