Aside from what Lisa says (which I completely agree with), they also require different water conditions. Whilst Oscar's are fairly adaptable, they do prefer soft acidic water - as they originate from the Amazon river (and tributaries). At least if we go by the "common African" - this meaning the Rift-lake cichlids (Lake Malawi, L. Tanganyika and L. Victoria are all rift-lakes and they have quite hard/alkalic water) that are most popular (due to their bright colouration and ease of breeding). There are other cichlids in Africa, Kribensis (Pelviachromis pulcher) being the most commonly available ones, and those are perfectly fine to go in a soft/acidic water.
Further, Oscars grow VERY LARGE, and therefore need BIG tanks to have a happy life - something in the order of 200 gallon is what you're looking at - although you may be able to get away with a custom built one that is slightly smaller.
For average Rift-Lake cichlids, a mid-to-large tank is fine, something between 40 and 100 gallon would be great, and smaller tanks can be made to work with carefull selection of which fish you pick and how you decorate the tank with regards to refuges, hiding spaces and visual separation. Like most things, a bigger tank doesn't actually hurt anything - just means that you can keep even more fish