The 37-year-old star claimed the 48-year-old movie mogul ''furiously masturbated'' in front of her when she was visiting a friend on the set of his 2004 movie 'After the Sunset' and entered his trailer to deliver a meal, after being asked to do so as a favour.
She told the Los Angeles Times newspaper: ''He walked out ... with his belly sticking out, no pants on, shrimp cocktail in one hand and he was furiously masturbating in the other.
''And before I literally could even figure out where to escape or where to look, he ejaculated.''
The actress let out a ''startled scream'' and raced out of the trailer, going straight to confide in the man who had told her to go in.
Discussing his response, she recalled: ''It wasn't a shock. It wasn't surprise. It was just, 'Ugh, sorry about that.' ''
Olivia confided in her sister, Sara Potts, who urged her to speak to a lawyer, but the legal expert advised her against going up against such a powerful man.
She said: ''That did leave an impact on me. How broken do women have to be before people listen?''
The 'Magic Mike' star previously wrote about the alleged incident in her 2010 essay collection but didn't name the director and she claimed later that year, the 'Horrible Bosses' filmmaker bragged about ejaculating on magazine covers which featured her.
She agreed to speak out after being infuriated by persistent false rumours they have been intimate.
She added: ''I've made specific, conscientious choices not to work with Brett Ratner.
''It feels as if I keep going up against the same bully at school who just won't quit. You just hope that enough people believe the truth and for enough time to pass so that you can't be connected to him anymore.''
Ratner's lawyer, Martin Singer ''vehemently denied'' Olivia's claims.
Meanwhile, model-and-actress Natasha Henstridge told the publication she was forced to perform oral sex on the director in his New York apartment after watching a movie with a group of friends, and alleged he blocked the doorway when she tried to leave.
She said: ''He strong-armed me in a real way. He physically forced himself on me. At some point, I gave in and he did his thing.''
The 'Red Dragon' filmmaker's lawyer confirmed his client had spent time with Natasha but claimed the actress was ''upset after learning my client had a girlfriend who he would not leave'' for her.
And Singer also disputed allegations from actresses Jaime Ray Newman and Katharine Towne that the director had made unwanted and aggressive advances towards them. branding their stories ''ridiculous'' and ''absurd''.
Four people working on Ratner's 'Rush Hour 2' described a ''sexually charged'' atmosphere, while Eri Sasaki and Jorina King both claimed they were offered speaking parts when the filmmaker made advances towards them.
The lawyer insisted his client had no recollection of the alleged incident, and dismissed claims from production assistant Kent Richards that a number of background actresses had received unwanted attention from the director.
He dismissed the account as a ''secondhand story about unnamed individuals.''
He also gave a statement from assistant director James M. Freitag, who insisted there had been no complaints while the movie was shooting.
James said: ''Any complaints of any kind including sexual harassment would be immediately directed to my attention. There were no complaints reported to me whatsoever.''
Singer wrote a 10-page letter to the newspaper in which he maintained his client's innocence.
He said: ''I have represented Mr. Ratner for two decades, and no woman has ever made a claim against him for sexual misconduct or sexual harassment.
''Furthermore, no woman has ever requested or received any financial settlement from my client.''
The director - who has been romantically linked with the likes of Rebecca Gayheart, Mariah Carey and Serena Williams - was also defended by five of his former assistants, David Steiman, Hopi Dobuler, Drew Sherman, Brett Gursky and Izak Rappaport, who all praised him as a boss and mentor and insisted they hadn't seen any inappropriate antics.
Steiman, who worked for the director from 1994 to 2004 said he would be ''shocked'' if anything untoward had happened.
And Dobuler said: ''I think he's great. ... He's a family guy.''
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