Water parameters aside, oscars and Mbuna still don't mix. Mbunas are bruisers of the fish world. Oscar's may be slightly aggressive but they are also big giant sissies. Just him being in with the mbuna as a new fish to the aquarium getting picked at can continue to keep him stressed out.
You may not see it but I would be surprised if they weren't picking on them. I tried once to put a jeweled cichlid in a mbuna tank with seemingly unaggressive mbunas. The jeweled cichlid was an inch bigger than every other fish in there and was a known tank killer. He went psycho years back and killed every fish in the tank. The Mbuna kicked his butt, causing him to hide, and by the time I realized how bad he was getting picked on he had pretty bad damage all over him. I had never seen them pick on him because they acted differently when people were in front of the tank.
My point is, that you really shouldn't mix south american cichlids with mbunas.