O'Reilly has denied the allegations, claiming that his celebrity status makes him vulnerable to such things.
Trump, 70, defended the anchor to The New York Times on Wednesday, stating that O'Reilly "shouldn't have settled". Because you should have taken it all the way. "He is someone I know well, is a good person", he added.
The number of ads that run on "The O'Reilly Factor" has been steadily declining as a growing amount of companies boycott the show amid the Fox News host's sexual harassment scandal.
The advertising exodus from Bill O'Reilly's Fox News show is having an impact on the format of the program itself.
"It's coming out of the mouth of a man who has said some terrible things about women", she said of Trump's support for O'Reilly.
California Democratic Congresswoman Maxine Waters, recently insulted by the embattled Bill O'Reilly as being inaudible because of her "James Brown wig", said Wednesday that the Fox News ratings king should go to jail.More news: Trump and Xi Jinping meet to discuss 'war today' with North Korea
CNN and Business Insider both have reporters who read the book and say that Old School contains some timely (if unsettling) material.
While O'Reilly may be so invaluable the network won't let him go, companies have launched a successful boycott of a host in the not-so-distant past. But this is Fox News, after all. She's supposed to be the one that's standing up for women.
O'Reilly's Tuesday program even beat offerings on ABC and NBC at that hour, in a rare victory for a cable network over broadcast television. He believes men can do whatever they want, and that women are there for the taking or are the problem.
During the U.S. election campaign, Trump had defended then Fox News chief Roger Ailes, who also was accused of sexual harassment.
Several more advertisers have joined the list of defectors from Fox News Channel's "The O'Reilly Factor", bringing the total to around 20.