Your tank is beyond overstocked. The 2 Gib plecs can overwhelm your bio filter by themselves. Water quality has put your Oscar in the condition its in. You have ammonia burn that has developed into bad infection. If your Ph is 7.5 or higher use Kanaplex. Under that level use Maracyn. You need to address the stock level in the tank or this will be a never ending problem. Start doing 3x weekly 75% water changes on the tank, and sell off lots of those fish.