OReilly isn't a straight Republican, but he is very conservative. Usually when I see him belittle or disagree with a Republican, it is for not being conservative enough. For example, I haven't watched him in a while (I don't avoid it, but I'm never around when he's on), but I would be willing to bet that he's belittled a republican on his show about the 700 billion dollar fix for being too "big government", which is a conservative standpoint.
Conservatism comes down to the argument between Big Government and Small Government, generally. This gets muddled when the "conservative" party wants laws against abortion, for example. But, yeah, the more conservative you are, the less national government interaction you want running society. We really shouldn't use "conservative" for "republican", but we do because it is convenient.
Again, and I can't stress this enough, it wouldn't matter if OReilly went easy on Palin or not. He tends to not go easy on anybody, I'd agree with that. Because he tends to shut down anybody who remotely disagrees with him as stupid. The thing is, his reputation (my reputation, Iago, my reputation! I've lost my reputation!) is conservative, and THUS the PERCEIVED outcome of the interview is that he went easy on Palin, whether or not he did. Rather, if you can get yourself to the opposite end of the independents and grab a few with an interview with a liberal journalist, and then get bonus points for "being attacked by liberal media", it is a strong move. She probably should have gone on BOTH, but her party, at least up to this point, has been trying to keep her from being too exposed for whatever reason. If you can only pick one, I think the move they made had solid strategical reasoning behind it.
I did call OReilly ultra-conservative, in a much earlier statement. Knowing my opinion about somebody may make it easy to misconstrue what I say. But that is not what I was saying recently. It's all about reputation.
That being said, I'm ultra-liberal (by the same logic that OReilly is ultra-conservative), because I believe that government involvement in regulating capitalism is a good thing, and I like limiting capitalism and the idea of government help to those who need, under restrictions. That being said, I don't agree with all (or even most) democrats on a lot of issues, and I am not a democrat. The same, but opposite, ideas can be applied to OReilly. He could be harsh on Republicans because he's farther conservative than them, just as I am with democrats.