There are several brands, mostly its a balence of cost vs availability vs will the fish eat it.
I started with Hikari gold, buy the small pellet size and a small bag. Oscar's grow real fast, an inch a month so you don't want to buy too much food and have it outgrow the food. There are other brands out there, some more available than others. Omega One, New Life Spectrum and on and on.
In the wild Oscar's eat everything from fish, to algea, shellfish, bugs,worms and all the stuff in thier bellies.
Imagine eating just one kind of meat your whole life, you can't get all the neiutrients the body needs doing this. Same for your fish just eating the worms all the time.