True, but think about what is contained in the guts of lots of the animals (I use animals as they eat fish, insects, and crustaceans) oscars would eat in the wild. Most all of the insects and crustaceans feed on plant matter, as do a large part of the food fish, which the "omnivorous" fish will then utilize.
Fishbase also has a good bit on their natural diet
here
They eat crayfish, finfish, insects larvae, planktonic invertebrates, benthic algae/weeds (Lemna spp., Echinochloa polystachya, Oryza spp. Paspalum spp.) terrestrial plants (fruits, flowers and seeds), terrestrial insects and worms (non-annelids).
So go ahead and feed them fruits and veggies if they'll eat them, I see no problem with it.