You have to figure out if your fish has lymphocystitis or Lymphoma - two very different diseases. The first is the most likely, as an incurable, non fatal viral disease. It's ugly, it disfigures the fish and there's nothing to be done.
Lymphoma is cancer of the lymphatic system, and while most of our diseases hit fish too, we can't diagnose that one. If a fish has it, it dies with us not knowing why.
There is some research suggesting dirty water or pollution contribute to the benign tumours, but there is no certainty - those studies are on wild fish, or food fish farms. Still, Oscars are messy, and I would just stay on the weekly water changes and not point at the poor guy, because he has to live with that growth! In all seriousness though, it's a thing that's there and will most likely stay there for the life of the fish. Clean water is all you can work with.