I've had a Green Terror and an Oscar together for a little over a year now in a 55 gal. and they have been fine. Get them both when they're young because the Oscar will quickly outgrow the GT, which I think discourages the mean side of the GT to come out. Sometimes my O will swim towards the GT but the GT will just get out his way, and all is fine. I think the GT knows his role pretty much! lol However once the Green Terror gets full grown you MAY run into problems, just have a divider handy. But GTs grow pretty slow, mine does at least. He's only about 4 inches right now after a year. They're all going back to fish store in a few months because I'm moving cross country, so I will never have to worry about the GT getting big and mean. But if I were you I would definitely take the risk, they are both awesome fish.