The "Charmed" star hit out at the media mogul over her former friendship with convicted rapist Harvey Weinstein, insisting she's happy people are "seeing the ugly truth" about Winfrey.
"She is about supporting a sick power structure for personal gain, she is as fake as they come," penned McGowan on Twitter. "I wish she were real, but she isn't."
"From being pals with Weinstein to abandoning & destroying Russell Simmons victims, she is about supporting a sick power structure for personal gain, she is as fake as they come. (sic)"
Record executive and producer Russell Simmons is fighting multiple allegations of rape and sexual misconduct.
Alongside a picture showing Winfrey kissing Weinstein on the cheek, the actress added the hashtag, "#lizard."
Last year (20), Winfrey stepped down as executive producer of Simmons' #MeToo documentary and insisted in a statement that "there is more work to be done on the film to illuminate the full scope of what the victims endured, and it has become clear that the filmmakers and I are not aligned in that creative vision."
She told The Hollywood Reporter at the time, "I want it to be known that I unequivocally believe and support the women. Their stories deserve to be told and heard."
The pair also clashed over Time's Up, which was founded during the #MeToo movement by Hollywood women to fight sexual harassment in the wake of the Harvey Weinstein scandal.
While Winfrey is a supporter of the organisation and reportedly donated $500,000 (£363,378), McGowan has spoken out against the movement.