The reason why Brian Williams got into deep trouble for lying about his war experience while Bill O'Reilly hasn't is that Williams and O'Reilly have different jobs, for different employers. Brian Williams was paid to sit in front of a TV camera and give viewers an ostensibly neutral, agreeable account of current events. When people began disagreeing with Brian Williams about his presentation of facts, loudly and in public, it hurt his ability to perform those duties for NBC.
Bill O'Reilly is paid to go on television for Fox News and say things that get the viewers upset, even if those things are false or nonsensical. So what if his experience in the Falklands war was bogus? So is his experience in the War on Christmas. The fact that people are calling him dishonest simply proves, from Fox's point of view, that he's doing his job.
14 hours ago