Typically Oscars tend to do better solo from my experience especially if you have one already. If you buy them and raise them together it can normally work out. I have 2 in my tank but they were raised together from fry and interact well.
It also depends on the size of the tank as well. A very large tank/pond can house multiple oscars without having too much aggression issues.
Tank size is the answer to your question.. if its a big enough tank Oscars can be together ...but i always found them better off alone... for me there was always a bully in pairs...