You see my world is big enough to include people who aren't Christian. It's big enough to include people who aren't religious. Why should I not convey good wishes to these people just because their faith differs from mine?
O'Reilly and others on Faux News have succeeded in making an issue of this over the last 10 years or so. Evidently unless you wish EVERYONE a "merry christmas" you're not a real American and you're giving in to some mysterious, always unnamed "them". I'm going to assume (however unwisely) that by "them" O'Reilly is referring to lib'ruls (as Molly Ivins would have put it), since that's the usual enemy on Faux News. He implies that lib'ruls everywhere go around stopping anyone anywhere from ever saying anything but "Happy Holidays".
What a bunch of hokum.
I worked for 17 years in healthcare in a university hospital at a large university in a large city. It would be difficult to find a higher concentration of lib'ruls in one place. And never, in all of my 17 years that I worked there, did anyone ever correct me if I wished someone "Merry Christmas" or "Happy Holidays". Now that I work at an insurance company in a different large city, I do get the occasional (very occasional, to be fair) conservative correcting me to say "Merry Christmas" to everyone. I do my best not to roll my eyes and point out that the person I'm talking to is Jewish or Muslim or has no faith. It's fascinating to see how quickly their self-importance deflates.
The bottom line is this: don't believe everything the media tells you. The only reason O'Reilly started this "campaign" of his is to create controversy so that he could improve his ratings. And CREATE controversy is exactly what he did, because there was nothing there before he brought up the subject.