Just nothing between the ears.
He has the annoying habit of talking as if he's caught his opponent in a lie or error but when he finally gets out his comment, you can even see in his eyes that he realizes he doesn't have anything, he's just TALKING like he does. I don't care much for O'Reilly (don't have anything against him, though), but I've seen this guy on his show, and OR seems to laugh an awful lot during those segments, like he wants to say "Will you get a load of this guy?"