The occasional feeder can actually be a very healthy supplement, provided care is taken to avoid disease, and they are not as nutritionally deficient as some think.
A vase is not a suitable environment for any fish, especially messy ones like feeder goldfish. Every time you place a dozen fish in that vase you are killing them through ammonia poisoning. Set up a 10 gallon tank for them using a simple glass tank and a cheap power filter, and purchase the fish in smaller quantities--half a dozen every two weeks is plenty for a single Oscar.
Because feeders are generally kept in overcrowded and poorly maintained environments, the use of them increases the risk of introducing parasitic or bacterial disease into your aquarium. Breeding your own feeders takes more patience and an extra aquarium, but it will provide you with an untainted source for supplement feeder fish.
As mentioned Oscars are not piscavores, and have no requirement for live fish in their diet in the first place. An avoidance of feeders altogether would not be a bad decision. Cichlid pellets are best as the primary source of the diet, and anything else should be considered a supplement, only to be fed as an occasional treat.
