Aside from the disparity in size (Oscars can get to 15 inches, Haps usually only reach 7 inches or less), there are several reasons to not keep them together. Many fish can acclimate to different water parameters, true, but Haps will prefer hard, alkaline water with a PH of about 8.4 or higher. Your oscar will do OK with this given time, but would prefer slightly softer, lower PH water. Their diets are totally different, and it's very difficult to feed individual fish that cohabit the same tank. Oscars need a meaty diet with a bit of veg; Haps need a diet heavy in veg matter, with a bit of meat. If your oscar doesn't get high enough protein or proper nutrition, you run the risk of hole in the head disease, and a generally unhealthy fish. If your Hap gets too much protein, you run the risk of malawi bloat, and, again, a generally unhealthy fish. Both can be fairly aggressive fish, and you really run the risk of the oscar killing your Hap. IMO, don't mix them.