O'Reilly was born on September 10, 1949, at Columbia Presbyterian Hospital in Manhattan, to parents William James, Sr., (1924-1985) and Winifred Angela (Drake) O'Reilly, from Brooklyn and Teaneck, New Jersey, respectively. He is of Irish descent, along with a small amount of English (Colonial American) ancestry.

In April 2017, The New York Times reported that O'Reilly and Fox News had settled five lawsuits against O'Reilly dating back to 2002. Previously, only the settlements to Mackris and Huddy were publicly reported; The Times reported that Fox hosts Rebecca Diamond and Laurie Dhue settled sexual harassment lawsuits in 2011 and 2016 respectively and junior producer Rachel Witlieb Bernstein settled with Fox in 2002 after accusing O'Reilly of verbal abuse. The amount paid to the women filing the complaints was estimated at $13 million. The New York Times also reported a claim by former O'Reilly Factor guest Wendy Walsh, who declined an offer from O'Reilly to go to his hotel suite and was subsequently denied a job as a Fox News contributor. Walsh appeared on The O'Reilly Factor for a few months after the hotel incident, and at one point asked producers for more airtime on the show.  After Walsh's complaint, 21st Century Fox hired the law firm of Paul, Weiss, Rifkind, Wharton & Garrison to conduct an investigation into that allegation; the firm also conducted an investigation into the allegations against Fox News chairman Roger Ailes, which led to his ouster from Fox.  After the five settlements were reported, the O'Reilly Factor lost more than half its advertisers within a week; almost 60 companies withdrew their television advertising from the show amid a growing backlash against O'Reilly. On April 11, O'Reilly announced he would take a two-week vacation and would return to the program on April 24; he normally took a vacation around Easter. On April 19, Fox News announced that O'Reilly would not be returning to the network. The program was subsequently renamed The Factor on April 19 and aired its last episode on April 21.  O'Reilly later stated his regret that he did not "fight back" against his accusers the way Sean Hannity did when facing the loss of advertisers around the same time.

When did O'Reilly start his career as a political commentator?