Bush, Burton, and Bethany Joy Lenz, as well as others from the show, wrote that they "have chosen this forum to stand together in support of Audrey Wauchope and one another" following Wauchope's statements made on Twitter.
"More than one of us is still in treatment for post-traumatic stress. 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," the letter read.
"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."
In a series of tweets, Wauchope said she and other women who worked on One Tree Hill, were often times subject to unwanted touching.
"To say we left that job demoralized and confused is also an understatement. One of the 1st things we were told was that the showrunner hired female writers on the basis of their looks. That's why you're here - he wants to f**k you," she tweeted.
She also talked about how Schwahn would allegedly show naked pictures of actresses he'd slept with to male staffers without the actress' knowledge.
"Men on staff were shown naked photos of on an actress he was having an affair with. Naked photos she didn't know were being passed around.
"Naked photos they didn't want to see. This is such a violation, both to the actress and to the men forced to look and participate," she wrote.
Wauchope also recalled a time when her writing partner was getting ready to leave for her wedding and Schwahn "closed the door of his office and asked her if she really wanted to go through with it".
"Maybe he could have a shot? Hahahaha we said. That's crazy. He's crazy. NO. He's abusive," she tweeted.
In the series of tweets Wauchope wrote: "I'm furious and sad and everything else for the women who have sat on that couch next to that man.
"And I'm furious and sad and everything else that years later I don't feel safe to be able to do anything real about this and that it seems to be happening all over this town."
One Tree Hill aired from 2003-2012 and followed the lives of a group of young men and women growing up in North Carolina.
Schwahn wrote and directed the series and worked as the showrunner for its eight-season run.
The cast and crew members who wrote the letter supporting Wauchope said they are all "deeply grateful for Audrey's courage".
"For one another. And for every male castmate and crew member who has reached out to our group of women to offer their support these last few days. They echo the greater rallying cry that must lead us to change: Believe Women. We are all in this together. "
Cast members Bush, Burton, Lenz, Danneel Harris, Michaela McManus, Kate Voegele, Daphne Zuniga, India DeBeaufort, Bevin Prince, Jana Kramer, Shantel Van Santen, and Allison Munn all signed the letter.
Crew members, Audrey Wauchope, Rachel Specter, Jane Beck, Tarin Squillante, Cristy Koebley and JoJo Stephens also signed the letter.
