Silversides is a common name used for any small schooling bait fish with silver or white sides on their body. In Australia they are regularly sold under the name of white bait, blue sardines and mulies. You can buy them from the bait freezer at any fishing shop and keep them frozen. When you want to feed the fish, defrost a couple and offer one at a time. If the bait is too big for the fish, cut it down to size.
You can buy frozen prawns/ shrimp from fishing stores too and these are dealt with in the same way as white bait. If you use raw prawn, the fish can normally digest the shell. If you use cooked prawn, remove the head and shell because the fish can't digest it properly.
If a fish is used to live foods, it might take a few weeks to accept dead foods. However, since the Oscar is being fed a variety of dry and live foods, it should take frozen (but defrosted) foods without any issues.
Oscars are predators and any plant matter they get is normally in the digestive tract of the fish they eat. White bait, blue sardines, etc are intact fish and have a full digestive system with food in from when they were caught.
If you feed the fish on fillets and don't use whole fish, then you can mush up some veges and mix them with the meat foods.
As for manufacturers making food for fish, they make a product that starving fish will eat because there is nothing else to eat. I must point out that not all fish food companies make crap food but a lot do.
Most dry fish foods contain flour (wheat and or soy flour). This is used as a binder and a filler to make the food more bulky but reduce the cost. Fish can't digest grains so any fish food with grains or flour made from grains isn't the best.
Fish foods can be made with whole fish or fish meal. Whole fish is the entire fish (head, guts, flesh, everything). Fish meal usually consists of scale, skin and bones. So read the directions carefully because a lot of companies use fish meal and wheat flour to save costs, and if you have fish food with these ingredients, you aren't giving your fish good food.
In addition to the above, some companies add things that appeal to humans. You might see vitamin a, b, c, d, etc, on fish food. Most of these are water soluble and also break down very quickly under damp or warm conditions, and are virtually useless in dry fish food. You might see ingredients like rosemary, garlic, etc. Again, people think they are herbs and good for me, but fish can't digest them. If the food has algae in, that is a suitable food for them. So check the ingredients carefully because most dry commercial fish foods sold in shops aren't the best for your fish.
If you want a good fish food, make and or grow your own.