I'd say remove the oscar, use the 55 gallon for a fair set of malawi cichlids, with extra filtration, of course.
My 50 gallon african cichlid tank has 10 melanochromis cyaneorhabdos, 6 yellow labs, and a bumblebee, which is what an african cichlid tank should be, a lot of fish to disperse the aggressive behaviors of the adults. But these fish are messy, and need a large level of filtration, at the moment, the tank is running a eheim 2217 canister filter(rated for tanks up to 160 gallons) and it's gonna get a second one running on it soon as well.
The issue with keeping the oscar in any mentioned situation does not bode well for any fish involved. Oscars grow fast. I got a young less than 1 inch oscar back at the end of november 2012 for my 75 gallon aquarium and it's already nearing 6 inches long. This is normal for them, but they won't grow that fast in a small, dirty tank with a lack of filtration while being fed an improper diet. African cichlids are primarily herbivororous, and if you give an oscar veggie food, they're not going to grow. If you give malawi cichlids carnivore food, they're going to get bloated and die. Nobody wins. This is one of many reasons why you never keep the two together. Not to mention the big differences in ideal water perimeters, the differences in temperament, and the differences in the way they like to be kept. Centeral and south american cichlidstend to like to have space to themselves. Mbunas are more heirarchal and like to be kept in colonies, not only to exercise social habits, but to disperse aggression amongst them so nobody is getting singled out and plucked off the list. The aggressive cichlid you rehomed was probably aggressive for being on top of the food chain. Chances are he's just going to be replaced by another.
While the stunting growth rate of being kept in a cramped, dirty tank may seem like a good thing for a person who wats to wait another 2 or 3 months to get a bigger tank for said oscar fish, the effects of growth stunting are not going to be magically reversed from being moved to a bigger tank with adequate filtration and diet, if kept in a small tank for too long, they'll end up with skeletal deformities and ultimately a much shorter lifespan, which isn't fair for the fish. I'd never put an oscar fish in a "grow out tank" state because of their quick growth rate. Oscars aren't necesarily rare fish, imo just find somebody with a tank big enough for it, and pick up another when you actually have the tank for it.