Just a little input...Oscars vary WIDELY in attitude. I've seen them mean, yet I have one that's a weiner. He's in with a JD and the JD shows no aggression. yes, they're in a 110 tall, but they hang out together with 3 parrot fish. the syno i have in that tank is the most aggressive. GT seem to be the same in attitude variation and it is important to start them out small...the oscar being the largest and established like you have...luck of the draw. I have proven that you can have larger fish in a smaller tank IF you are willing to take on the additional care required. Over filter, water changes of 50% a week (no this will not kill them). Because of their eventual size, the tank should be bigger to keep any aggression down and they should be well fed. keep us posted