The Girls Next Door stars Holly Madison, Kendra Wilkinson-Baskett and Bridget Marquardt all took to Twitter to give their condolences, along with Hef's new wife, Crystal Harris. 

Holly wrote, "Rest In Peace Mary."

While Kendra tweeted, "RIP Mary O'Connor. She was an amazing person who helped me through so much. But I know she's happy with her boo Captain Bob. :)"