"I've asked her to marry me, again, and she's agreed," O'Neal told Barbara Walters for "20/20."
"We will as soon as she can, say yes," O'Neal said. "Maybe we can just nod her head."
Fawcett, 62, is terminally ill with anal cancer and is nearing the final stages of her 2 1/2-year battle against the disease that spread to her liver and left her bald and bed-ridden -- but O'Neal said he has never wanted to marry her more.
"I promise you, we will," he said during the interview that will air on ABC on Friday. "Absolutely."
The actress is in the hospital again as her condition worsened though a source told Access Hollywood that she could leave the hospital today.
O'Neal, 68, has implored the former pinup model and "Charlies Angels" star to marry him for years -- but she has always resisted. The two have dated on-and-off since 1980 and have a 24-year-old son together.
"I used to ask her to marry me all the time," O'Neal said during the interview. "But ... it just got to be a joke, you know. We just joked out it."
In May, O'Neal said he had never wanted to marry the actress more than ever.
"She's still a little bit hesitant. I'm working though, I'm working," he said. "You know she's been married, I've been married. She's said, 'We have a good thing.' But I said, 'You said that 30 years ago, shouldn't there be a sea change?' Maybe there is, you never know."
Though no plans are officially in the works, O'Neal joked he would dress "like a gigolo. You know? And a little thin mustache and slicked-back hair. I don't know. We have fun, we tease about that."
Farrah's heartbreaking struggle against cancer was chronicled in doctor's visits and hospitalizations and captured on a video diary that aired on NBC in May.
"It's a gruesome story," Ryan said. "It's a terrible story but sometimes out of terrible stories come a light and a hope."