Flakes will not work.
Feeding your Oscar fish on a healthy diet is very important indeed. Oscar fish can be susceptible to various diseases so amongst healthy tank conditions, a healthy and suitable diet is a good start to keeping your Oscar healthy for many years.
Like a lot of creatures that prey on live food, Oscar fish have got very big eyes. They rely purely on their excellent eyesight to locate and hunt food. Their eyesight is so good, they can even see things behind them because of the way the eyes protrude. Oscars will feed at any depth so they will readily accept floating, or sinking food.
In the wild the Oscars diet consists mainly of insects and small fish. It is important that you feed your Oscar on a healthy and varied diet. An unhealthy diet could well lead to your Oscar developing disease or picking up parasites. If you want to try and replicate an Oscars natural diet than you could harvest insects and critters from your garden. Beetles, spiders, cricket's, worms, will all be a tasty and healthy snack for your Oscar fish.
Two of my favourite brands are made by Hikari and Tetra. These being Hikari Cichlid Gold which has a colour enhancing agent added. I also always have a supply of Tetra DorImin. They come in various sizes that should suit the very smallest, to the very largest Oscar. If you have a medium to large Oscar, I would suggest that you always purchase a larger size in these products.
Hope this helps!