Hi Flowers,
Oscars grow very fast. They can do one inch per month. Their growth of course depends largely on how you feed them, what you feed them, and the quality of the tank water. For optimal growth ammonia and nitrates need to be kept down and of course nitrites should be eliminated. They get into pretty vicious fights when they start maturing, at about 5 to 6inches or so at which time they'll need some good space to run away from each other. I had 6 in a 65gallon when they were at this size. I'd hear like splashing all the time and sometimes saw a lot of skin ripped off all of them but one, the biggest/meanest male. I then separated them to other tank. And yes, some will grow faster than others but I wouldn't worry too much about them killing each other, oscars are not really very aggressive like red devils etc..just give them enough space....
Joe
www.cichlidfish.com