Oscars really won't grow to their full size in anything under 125gallon long, and thats asuming minimal companions.
Many will say one in a 55 is good, but that is a false statement. That is like keeping a silver arow in a 125 gallon tank, the fish will never reach anything near a healthy adult size in such a tank. A 75 gallon will only function for the first 1-3 years, after that you will really need to start looking at doubling your aquarium realestate, especially to keep a pair.