Very bad idea. I have both fish,(full sized) and would never put them together in any size tank. Flowerhorns are some of the most agressive and territorial Cichlids on the market. Unless you raise them young with other agressive Cichlids, such as Blue Nile Perch, Flowerhorns, Red Devils, or Wolf Cichlids(in a massive tank, 300+ gal) forget every having tankmates. Your Oscars would be meat for the beast. Best of luck.