My psittacus recently ripped its whole side open on a piece of branchy driftwood. It had a huge vertical gash down the back of his eye that went from near his dorsal fin to his gills. The flesh was exposed and you could actually see inside the fish. He didn't eat for two days, but I started doing large daily water changes to keep the tank really clean and he healed on his own over a week or so. There's still a needle-sized hole behind his eye that refuses to close up, but otherwise he's completely healed and acts normal again. Maybe your oscar experienced something similar.