The obvious choice to me is Ryan Spangler. He can be a beast inside when he's not worried about getting into early foul trouble. It's easy to forget what he was like in his first season at OU. He was fearless and relentless in scoring down low. That changed some when the early foul trouble bug caused him to back off some. I think at least to some extent that was by design, because he has always been so important to the team's success.
My second choice would be Dante Buford. He is a very versatile player, more like Romero Osby than anyone else I can think of to use as a comparison.
I also agree with what usedtobe1 said about Manyang. If he is healthy, and that may be a big IF to start the season, I think he will give us an offensive post presence down low, even if it's mostly on put backs and jump hooks in the lane.
Maybe this is wishful thinking on my part. But I'm not as concerned about our scoring down low as some seem to be.