"Mary passed away today. We loved her more than words can say," the Hef tweeted to his nearly 1,300,000 followers. O'Connor, who worked with Hefner for more than 40 years, was a familiar face to fans of E!'s hit show The Girls Next Door.
The three former stars -- and Hefner's then-girlfriends -- all similarly took to Twitter to share their condolences for the Playboy mansion's loss.
"RIP Mary O'Connor. She was an amazing person who helped me through so much," Kendra Wilkinson-Baskett posted. "But I know she's happy with her boo Captain Bob. :)"
"Thank you for all the sweet messages," Bridget Marquardt wrote. "Mary O'Connor will be missed. #RIPMary."
Holly Madison kept her sentiments succinct: "Rest In Peace Mary."
Hefner, 86, has previously credited O'Connor with helping to orchestrate his reunion with wife Crystal Harris, 26, after the two famously called off their engagement in June 2011. The couple married in a private ceremony this past New Year's Eve.
"It's Mary O'Conner that brought Crystal Harris back," Hefner tweeted on June 1, 2012, after the pair reconciled. "Crystal was miserable & said so. Mary told Crystal to write & tell me, which she did."
Harris also expressed her sadness over O'Connor's death Sunday.
"Hef's long time secretary & my dear friend Mary O'Connor has passed away," she wrote. "It's a very sad day here at the Playboy Mansion. We love you Mary."