I am a huge fan of pike chiclids, they might bully your oscar a little but the oscar should be able to hold its own as long as you dont get too many fish in there. You would only be able to get 1 or 2 though as most get to 18". Also if you get the pike dont put in silver dollars because they will be killed. I had 3 silver dollars at 3" and I added a 4" pike, the next morning all of the silver dollars were dead 
