Oh lordy. K, here we go
First, a 75gal tank is too small for a frontosa adult, unless you plan to keep
only the frontosa in the tank. That being said, Frontosa should be kept in groups, they live in colonies and are much more comfortable and happy that way. Frontosa colonies need at least a 6ft. tank. However, Frontosa should not be kept (solo or in colonies) with any of the fish you currently have living with it (except
maybe the yellow labs, but that's pushing it). Frontosa are from Lake Tang, and require a much higher pH level and very, very hard water, much more so than any of the other fish. Also, Frontosa are very shy and timid fish, and the mbuna you have in that tank aren't a good match because of how aggressive and fast they are - the Front won't be able to compete for food forever, and you may give the poor guy a heart attack with the mbuna zipping around him
And lastly, Fronts easily reach 12" or bigger, and they will eat or try to eat anything they think they can fit in their mouths - which is pretty much every other current fish in the tank.
Moving on from the Frontosa, you have four mbuna in there - the lab, kenyi, trewavasae, and Mpanga. These fish are not suitable to live with an Oscar. Mbuna are from Lake Malawi, Oscars are South American. They have completely different water parameters for one thing. And yes, an Oscar will likely eat or kill most of the fish in the tank as it grows, as they stay much smaller than an O.
And lastly, even without taking consideration of how an Oscar shouldn't be mixed with the existing fish in your tank, based on room alone you couldn't fit an Oscar in there. Really, an adult oscar should have a 75gal tank to himself if you ask me, with maybe some dither fish (giant danios, silver dollars, etc) and/or plecos or large-ish catfish.
