No room for any more Oscars. You got good filtration with that big filter. Get your new tank filled, set the filtration up and let it stand for a couple of days to give the water a chance to warm up. I would then go and get some small fish to cycle the tank. You can follow the instructions on the "how to cycle a tank " page. I recently used a product called stresszyme and even though there's a little of controversy about these type off products, I have to say it did work for me. I would probably only is it on small fish such as tetras and guppies and see how they get on. Just keep check on the ammonia, nitrite and nitrates. When you start seeing a reading of nitrate, you know that bacteria are present.
How big are your Oscars at the moment?