Let me first say, I like K.O, he's a good kid, a hard worker, and is a useful piece. However, I like him off the bench for his scoring and passing ability against the second team.
But using him as a starting center is a disaster. He is a poor defender in general at this point in his career. Seeing him being abused at the center position is painful. Opposing teams go right at him, he has lead feet, can't jump, and poor reaction. His stats are deceiving, he's a good position rebounder, so he gathers a few rebounds, and decent shooter, so he scores. But he gives up a ton of points at the center position.
Now that said, Zeller is not Bill Russell either, but I feel he is the better choice until we can get a center the other team will respect in the middle.