I disagree, oscars can thrive, breed, and live long lives in water that is not acidic. Most oscars in the hobby now, like those bred in FL fish farms are most likely not coming from similar water to those in the Amazon. HITH is more likely to occur due to dietary/nutriotional deficiency and high nitrates more than whether they are in lowe pH, soft water. The problem will be diet, because the mbuna will end up getting bloat from the high protein diet the more carnivorous oscar prefers. Either way, the mbuna should definitely not be mixed with oscars as you said!