It'd be great if oscars would stay at 5-6 inches but this things grow. Had a few oscars in the past, I found them easy to keep, fun to own, they almost interact with you. They outgrew just about any tank I put them in, but then again I never had a 100g tank, 75g being the biggest I could give them. Having an oscar in a tank is all about the oscar and nothing else, any other fish will most likely become food and another oscar in a 100g will be tough, toss of the dice. They are messy eaters so frequent water changes a must.
If you like large fish it could be for you.