the main issue with putting an oscar in a mixed mbuna tank is basically diet differences. There are other issues, such as an oscar being able to eat smaller mbunas, and the one that would concern me most is that mbunas will mercilessly harrass an oscar.
If it is mixing an oscar will some large haps in a large tank, then it shouldn't be a big issue as long as the o is acclimated to the water conditions the malawis prefer