Feed a good quality pellet as a staple with a few crickets as treats, if you want to keep them entertained get some floating plants and put the crickets on them, the oscars will work out how to get them off
I stopped giving them pellets because my oscar died from it, I believe. I forget what it's called but apparently they can't always break down the food when it's very dry like with pellets and it can cause a blockage. I'm not a hundred percent sure that's what killed him, but from the signs and everything else that's my best guess. So with my new Oscars I have been mixing spirulina with flakes and making my own pellets that have much more moisture content.