It will end up costing you a fortune to feed brine shrimp, also unless the shrimp are large your oscars will ignore the shrimp as not worth the effort. Also a varied diet is best as fish can become hooked on one type and not eat any other sort, and then become bored with the one they are hooked on
There are many different type of food to feed to oscars, pellets, maggots, worms, frozen prawns, lance fish, meal worms, any insects you can hold off and any small fish from the fishmongers
Scatter the food all over the tank so the smaller fish gets a chance to eat