Yeah, quarantining Oscars is no easy business. Best to withhold food, or just feed a pellet or two per day. You really need to prevent waste buildup as much as possible, even in a strongly cycled tank if it's small. You'd need something like a penguin 350 or a medium-sized aquaclear just to keep up with an adult Oscar in something like a 20 gallon tank for a few days, and that's minimum. Add sponge muck from established tanks freely to help destroy ammonia, and keep water changes to a maximum of about 75% every two days. In my experience, Oscars have not done well with daily water changes, as much as I tried to convince myself otherwise. They seem to thrive on that fence between stability and stagnation, where there is no ammonia or nitrites, and nitrates are as low as possible (not exceeding 20ppm), but where water changes aren't too frequent or large. Hence the large tank sizes required to dilute their waste.
But as for the pouting, yes. Captive strains seem to pout relentlessly. My wild Oscar has never pouted, though. I can toss him into a bucket and gently scoot him into a new tank and he just looks at me like, "What? No worms in here? Call the maid!".
