Fluvals are the best, and are very often used for turtle tanks, which are even messier than Oscars. The only problem with fluvals is about 1 in every 10 of them need to be replaced within 6 months. If you get a good one, it'll last for many years, but otherwise it could be a waste. The second best filter is the Rena XP3, similar to the Fluval, but less media room and they won't need to be replaced as often. Be warned, both of these filters have VERY powerful intakes, especially the Rena. I think there's something covering the intake on both filters so no fish get sucked up, but that piece has been known to come off, and many turtles have died from getting a head stuck in there, or a foot. I'm not sure how this would affect fish, though.