One of Vanderbilt University's longtime employees says she's been bullied and viciously mocked by co-workers -- using words like "freak," and "gross" -- after she underwent transition surgery.

Olivia Hill is suing Vanderbilt for the abuse she says started back in mid-2018, when she'd informed her employer she'd been diagnosed with gender dysphoria ... and, therefore, needed to transition from male to female. She says she soon learned a direct supervisor was referring to her as "it," "trans freak" and "weirdo."

Olivia -- a U.S. Navy veteran who's worked at Vandy for 25 years -- says that sort of reaction made her second guess getting the surgery, and she even delayed it until February 2019. In the lawsuit, obtained by TMZ, she says the abuse was even worse when she returned to work in May 2019 -- some people would leave rooms as soon as she entered, while others made crude remarks.

According to the suit, those comments included a bigwig commenting directly to her about her big feet looking strange. She alleges he also asked if she shaves her chest now, and added ... "Ugh, the thought of you shaving your tits is gross."

Olivia claims in the suit another employee asked if he could "be the first to use her new vagina."

Interestingly, Vanderbilt touted Olivia's transition ... even posting a video where she tells her story. However, Olivia's attorney, Abby Rubenfeld, says this only highlights the university's hypocrisy -- "The way Olivia was treated violates federal and state law, and is inconsistent with Vanderbilt's own policies and public presentation as allegedly being a model of LGBTQI tolerance and inclusion."

Olivia says she informed her supervisors, but got no relief, and even filed complaints with the University and federal employment agencies. She says the school placed her on paid administrative leave, and says she's been passed over for promotions.

She's suing Vanderbilt for damages, including lost wages and pain and suffering.