If you raise them together when they are small, you could try and get a pair of Oscars. Personally, I would only keep one Oscar in the 75 gallon. South American cichlids are prone to head and lateral line erosion. Lots of speculation, but most feel it's brought on by poor water quality. Oscars are really pigs when it comes to generating an inordinate amount of waste. If you do manage to shoehorn in another large cichlid, make sure to stay on top of your tank maintenance with water changes and siphoning.