Both Bill Clinton and Bill O'Reilly have exhibited terrible behavior. No one is letting either off the hook.
But there are two key differences: first, it's 20 years later. People were (unfortunately) more tolerant of this conduct in the 1990s and before. However, it's 2017... there is no excuse for not only continuing to be a sexist asshole, but to receive the defenses of your employer, your viewers and the President in the process.
The other part is simply that there's a level of hypocrisy in O'Reilly that wasn't present with Clinton. Sure, Clinton had to do the usual politician's song and dance to pretend he was devoutly religious (since, sadly, it's hard for agnostics/atheists to get elected), but his presidency was not staked on sex-oriented morality. However, O'Reilly is a figurehead for Fox News, which slavishly toes the Republican line on a prudish kind of sexual morality. He's supposed to be the shining beacon of restraint, not classic sexist horndog behavior wrapped in a different suit.