From your experience, at what age does the oscar growth start to really slow down? I have had my oscar as a baby for 7 months now and its about 8-9 inches including the fins.

But for the past 2-3 months he hasnt grown at all....... is this normal?

They start getting thicker. I have one about 14 months and it grew in length fast then it started to get girth.

I would say you are probably on track. At about 9 inches you will see the growth rates slow down quite a bit.