If it's not an intestinal blockage, it could be internal tumors caused by a bacterial or viral infection. My 5 year-old Jack Dempsey pictured in my avatar had this. He had tumors on both of his sides right by his pectoral fins, and his side is a bit uneven, though not to the extreme of your Oscar. My JD continued to eat, but I did not like the tumors, so I took it to the vet for diagnosis. 1 office visit, 1 fishy tumor-removal surgery, 2 weeks of force-fed oral antiobiotics, and $520 later, the tumors are removed, but his side is still uneven, and the vet is still offering more surgery for God knows what price. The Jack is still alive, eating, and romping around with his female. I've decided in my case that what happens happens. In your case, I would try the peas, or try a pellet with more fiber/ plant matter concentration, and other than that just enjoy the fish while you have him. Vets that specialize in fish are few and far in between, and they will wring the very last dollar out of you. I'm glad I got the surgery this once, because the Jack Dempsey is like my child, but I would not ever do that again for any other fish I own. Hope that helps, and I hope your fish has more years ahead of him.