senegal or an ornate. If you go with the senegal your oscar should be safe, but like dysfuntional said, if the poly is small the O might mistake for a wormy treat. Maybe grow the poly out for a bit in the 55 (which from what I've read would house a senegal for life) and then once it fattens up introduce it to your 150. With the ornate, it might be a little snappy towards the oscar at first, but should settle down. When I got my ornate it would try to bite all the fish in the tank, but after a week it just ignores them all now.
I got my 14" ornate cheaper from my LFS ($42 tax included) than what the Petsmart here charges for their 4" senegal's. Wonder why the Canadian Petsmart prices are so different than the US... They're charging 49.99$ (+tax) for baby senegals, not even albinos. I was able to get my albino senegal for $10 at around the 4" mark off someone from this board.