It's in O'Reilly's lawsuit. Her boss at CNN was under sexual harrassment allegations. She learned how to bring these charges from CNN.
In his complaint:
Then in January 2004, Mackris ultimately decided to leave the show to accept an even higher-paying position with CNN, Fox’s major competitor. A few months after leaving Fox, however, Mackris spoke with O’Reilly and expressed unhappiness with her new job at CNN. She said that her boss at CNN had been terminated for sexual harassment, and asked O’Reilly if she could return to The O’Reilly Factor at the higher salary she then earned at CNN. Mackris returned to Fox in July 2004 as an Associate Producer for The O’Reilly Factor at a salary of $93,200.
Prior to September 29, 2004, Mackris never complained to anyone in authority at Fox about sexual harassment by O’Reilly, despite having received and being well aware of the clearly written harassment prevention policies and procedures of Fox and its ultimate parent, The News Corporation Limited (“News Corp”).