The Charlie's Angels star was struck down with anal cancer in 2006 and O'Neal was a constant presence at her side as her condition deteriorated.
The Love Story star, who is fighting stage two prostate cancer, detailed his heartbreak over Fawcett's failing health in his diaries, which have been made into a new book, Both Of Us: My Life With Farrah, and serialised in Britain's Mail on Sunday.
In the tome, O'Neal tells how he planned to marry Fawcett in St John's Hospital, but the priest who arrived to oversee the ceremony ended up administering the last rites instead.
He writes, ""After the priest leaves, I lie down next to her, wrap my body around her to keep her warm, and then take her hand. I can feel a steady pulse... I caress her hand for hours. Her heart refuses to quit.
""I'm left alone with my love. I take her hand. I can still feel her pulse, but now it is fluttering. She's trying to let go. Her heartbeat slows, then disappears. On the morning of June 25, Farrah slips into eternal sleep.""
O'Neal also shares his shame over the physical altercations he and Fawcett endured during their tumultuous relationship: ""Neither of us possessed the emotional discipline to say: 'Wait a minute, this isn't normal, we need help.'
""You have to remember that I'm a trained boxer... If someone is coming at me with fists flailing, my instinct is to block the blows, which is what I did with Farrah... I didn't see my behaviour as aberrant. In reality, most of the time it wasn't her I was lashing out at, it was my kids, the world, Hollywood, my agent, you name it. I'm a moody guy.""