"I don't know what I'll do without her," Ryan said.
Fawcett, 62, has struggled with anal cancer for two and a half years and her health began to rapidly deteriorate about "five or six months ago," O'Neal told the "Today" show.
"She said she's had a terrible time the last year. Terrible time," O'Neal said on the "Today" show. "There were times she got terribly ill and began to vomit. ... She didn't care. She didn't care. She was possessed."
Farrah and Ryan's son Redmond has struggled with his mother's worsening health as he remains jailed on drug charges, his father said. The 24-year old was recently allowed a three-hour visit with his mother.
"When she lost her hair, he shaved his head," O'Neal said. "He wanted to see her. He wanted to hold her. He wanted to apologize. He's so full of shame for his mistakes."
The "Charlie's Angels" star expressed doubt that she could beat the cancer that had taken hold of her life a few weeks ago.
"She asked me once, 'Am I going to make it?' She asked me that a couple weeks ago. I said, 'Yea, sure, you'll make it. And if you don't, I'll go with you,'" O'Neal said.
"She's not afraid. I've never seen fear at all," he said.
O'Neal is featured on Farrah's documentary "Farrah's Story," a video diary that chronicles the star's fight against cancer.