I got the big oscar from a friend so he was already large, about a year later I got the red albino oscar as a baby (about 5 inches) and the 2 got along ok, until the albino reached about 8 inches
then I had to seperate them. The little guy beat the crap out of the big one.
So far my tanks have faired well. and I wish you luck in choosing buddies for your oscar.