You asked minimum size and IMO the minimum size is 75. The reason I say I like 90 is 15 more gallons of water. Ideally I like to see an adult Oscar kept in a 125 but that's not really always practical. It is more difficult to keep water quality up in a smaller tank. So more maintenance is required. Oscars are prone to Hex in less than ideal water, that means nitrate levels must be kept below 20 ppm and ideally below 10 ppm. Larger tanks are more stable and parameters will fluctuate less. The old saying goes we keep water not fish is especially true with Oscars. If you keep the water clean the space is less of an issue. However bigger is always better but if you put in the time and keep the water quality up a 75 will work, It's not ideal but it is the minimum size you can get away with.