My primary point about Bill O'Reilly is the counterintuitive one. In a perverse way, he encourages and enables the worst impulses of the left. Yes, you read that right.
The obvious point everyone makes about O'Reilly is that he is/was the standard-bearer for Fox's "fair and balanced" bullshit. He is a conservative Republican whose primary goal has been to advance the Republican Party's electoral interests, and the policy goals of the social wing of the conservative movement. Very few people are straight-down-the-line liberals or straight-down-the-line conservatives, and O'Reilly loved to play up any deviation he had from the Limbaugh crowd in order to pretend that he was a non-ideological, non-partisan, muckraker-style guy who just called bullshit on anyone who deserved it. But... no. Anyone with a brain could see through him.
So, he just helped push people further to the right, right? Give people conservative-leaning news, tell them it's the straight truth, and you will reinforce their biases, right? Maybe, but the thing is that even though O'Reilly's ratings were comparatively good, that's only compared to other cable news programming. Yes, he got better ratings than whatever crap was on CNN or MSNBC, and better ratings than what you should be watching, which is The Expanse (holy shit, is that show great, right?!), but it is still a very small group of people who watch, in raw numbers. And even among them, measuring the effect is difficult.
I am concerned about O'Reilly's broader effect, and I've written about the effect of a complex media environment on the signals that voters receive. Here's the problem: Politician says X. Journalist calls bullshit. One of two things is true. Either the politician is full of shit, or the journalist is a shill for the opposing party. If the voter has strong "priors" that the parties are basically equal, and knows that the media environment contains partisan shills, many of whom don't even admit what they are (e.g. O'Reilly), then when a journalist tries to call bullshit on our politician, the rational thing for our voter to do is to conclude that the journalist is a shill for the opposing party. Even if the politician really is full of shit. That's what comes out of the math in the paper linked above.
See the problem? The existence of shills for Party A makes it easier for Party B to get away with shenanigans because any time Party B is called on something, they just accuse the journalists of being shills for Party A, even when the journalists are just honest watchdogs trying to do their jobs.
So, Bill O'Reilly was a shill for the Republicans, pretending otherwise. He at least had the potential to encourage Democrats to lie and cheat, and accuse any journalist who called them on it of being thinly-veiled Republican shills, thereby getting away with it. After all... Bill O'fuckin'Reilly!
Empirically, has Bill O'Reilly had this effect? Not as much as effects running in the other direction. Republicans have been playing the "liberal media bias" game for much longer. They really started ramping it up with Nixon, and it takes time. Democrats would do well to learn some lessons from that process, strategically. It is worth keeping in mind, though, that few people actually watched O'Reilly. His effect on the media landscape, though, is what matters.