The 35-year-old Burton, who played Peyton Sawyer on the WB/CW drama, told Variety that she fought against Schwahn's attempts to overly sexualize her character in the first two seasons. She says that led her to being labelled "difficult."
During production on the third season, he began acting inappropriately, even in the presence of his wife. She also claims Schwahn spread false rumors about being in relationships with the show's actresses.
Burton told Variety Schwahn kissed her twice on the lips against her will. She also says he inappropriately touched her, while his wife was present.
In one case, Burton claims Schwahn forced himself on her during a party with cast and crew members after filming a 2007 episode in Texas.
"I'm at the bar waiting to get drinks for the hairdresser and me, and he leans over and he kisses me in front of everybody," she told Variety. "Right on the mouth. It was something I had to get out of. It wasn't a peck. It wasn't 'Hey, sis, how's your day?' It was a boyfriend kiss."
Burton appeared on the show's first six seasons, but turned down a big raise to stay.
"I didn't want Warner Bros. to view me as a problem, because they had been so supportive" she told Variety. "I wanted to work at Warner Bros. again. I'm working at Warner Bros. now."
The actress came forward after writer Audrey Wauchope took to Twitter on Nov. 11, accusing Schwahn of sexual harassment and making the writers room uncomfortable for female writers. On Nov. 15, E! Network suspended Schwahn from The Royals, another series he created.
Burton and Wauchope are not the only people who worked on One Tree Hill, which ran from 2003 to 2011, coming forward to describe an abusive environment on set.
Writer Michelle Furtney-Goodman told Variety of one incident when Schwahn forced her head between his knees, put a soda can on her head and joked about oral sex. Another actress, who spoke anonymously, said Schwahn pulled her between his thighs on set.
On Nov. 12, One Tree Hill cast and crew members released a joint statement, in which they claimed some of them were put in "positions we felt were physically unsafe."
"Many of us were, to varying degrees, manipulated psychologically and emotionally. More than one of us is still in treatment for post-traumatic stress," the letter read. "Many of us were put in uncomfortable positions and had to swiftly learn to fight back, sometimes physically, because it was made clear to us that the supervisors in the room were not the protectors they were supposed to be. Many of us were spoken to in ways that ran the spectrum from deeply upsetting, to traumatizing, to downright illegal. And a few of us were put in positions where we felt physically unsafe."
Last month, after the Harvey Weinstein scandal broke, Burton also said Ben Affleck groped her on MTV's TRL in 2003, of which he has since publicly addressed and apologized for on The Late Show with Stephen Colbert.