Thanks for the additional information. You have a couple of problems going here. The first, and I what I think is the most critical, is when he was allowed to gorge himself on goldfish. Oscars just never learn when they should back away from the table! He probably won't eat for a week or more, as he is trying to digest it all. Hopefully he will!
To help him do this it is important that you are making water changes. I would take a little more water each day, and work your self up to at least 50%. When when you change 20%, you are only diluting the problem, you need to be able to help the filter by removing as much of the bioload as you can.
Regarding aggression... Your tank is really too small, and the first and best solution is to see if you can get a bigger one. If you can't then I would move the pleco, and just let him be by himself. He will be much happier, and the pleco is contributing to the bioload on the tank. These fish often do much better solitary, and cichlids are very territorial, and since he is so large it is easy for him to either eat or make life rough for any tank mates. Don't worry about him being alone in the tank, he will be elated to discover he has no one to chase out of his territory.
I can tell that you like this guy, and I do think you have been trying to take good care of him. Get him back on the pellets or sticks, and remember that you can control his diet, and just give him a little like no more than 6 sticks, twice a day, and see how that goes and he needs to eat it right away, if he doesn't then skip the next feeding. Now he is probably going to stick his nose up at them after that wonderful goldfish buffet. So stick your guns, he will figure it out. Live fish are really not good for oscars, and it is an easy way to introduce diseases in your tank.
Once he starts feeling better, and with a decreased diet, you won't have to change the water every day, but because of the size of your tank, you'll probably need to do at at least every 3 - 4 days.
We didn't talk about your filter, and perhaps adding an additional one would also help keep the water cleaner.