I'll cut straight to the chase, I think your Oscar is terminally ill. My very first Tiger Oscar had these exact symptoms. He stopped eating, started swelling and after around a week and a half, I decided enough was enough and I euthanised him. I did try him on a couple of medications that were supposed to treat internal bacteria infections but they didn't work. He was also lying on his side a lot. I could be totally wrong and he could be severely constipated. However, the fact that he is not eating is not a good sign at all. In cases like this, it is very difficult to treat the fish because if it won't eat, you can't put medication in its food, or feed them on things like peas that can help with constipation. You could try some kind of medication that treats internal bacteria infections, because that's what it sounds like to me. However, I'm not going to give you false hope.
I really think you should prepare yourself for the worst. What I think you should do now is get on to professional with it straight away, try and find one who knows a little bit about fish. We are not trained professionals on this site, nor are many other people on other forums. A trained vet will know of different medications that may be able to be used on animals that have stopped eating. A lot of these medications will probably only be obtained by prescription, they won't be off the shelf medications.
