Your tank is seriously overstocked, you need to rehome alot of those fish are they are not suitable for your tank. Im assuming the tank isnt cycled and therefore the fish are undergoing ammonia and nitrIte poisoning, which if it doesnt kill them, will shorten their lifespan considerably.
The Oscar is an aggressive species, surely you should have known that prior to purchase, although from what I can see he is still a juvi and would unlikely be the culprit for any deaths yet, unless it was of small fish he could fit in his mouth. However, having said that, all fish will turn aggressive when housed in inappropriate conditions and undergoing poisoning, strange huh.
You cannot keep the poor fish in a bag, stress alone will kill him. I dont believe there is no fish store that will take it from you, sure they may not give you any money for him, but lets face it, most people would say the money wasnt important, the welfare of the fish was.
IMO you need to re-home 2 of the acaras, the oscar and the black ghost knife fish (who for lifelong health needs a 5ft tank). If you keep the current fish, the oscar will no doubt kill pretty much everything else in the tank as he grows, maybe cept the bala's and clown loach, who again need larger tanks, those he cant eat, he will maime.
In the meantime, do daily water change of a minimum of 50%, sometimes 2x daily, until you can get ammonia and nitrIte under control. Do you even have a test kit?