"I've asked her to marry me, again, and she's agreed," O'Neal, 68, told Walters.
"We will, as soon as she can, say yes. ... Maybe we can just nod her head," he said, with a laugh.
Fawcett and O'Neal began dating in 1980 and lived together with son Redmond. The two never officially tied the knot, but not for O'Neal's lack of trying.
"I used to ask her to marry me all the time," he said. "But ... it just got to be a joke, you know. We just joked about it."
But O'Neal told Walters that he was confident the two will finally get married. "I promise you, we will. ... Absolutely."
O'Neal also said he has his wedding outfit prepared, telling of the couples' ability to make each other laugh.
As for how he will dress for the ceremony, O'Neal said, "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."
Barbara Walters' complete interview with Ryan O'Neal, as well as Farrah Fawcett's doctor and others who are closest to her, will air on "20/20" Thursday at 10 p.m. ET.
Walters interviewed Fawcett in 1984 when she was pregnant with Redmond, her son with Ryan O'Neal. On why she never accepted O'Neal's many marriage proposals, Fawcett said then that he was "too bossy."
In 1997, after 17 years together, Fawcett and O'Neal parted ways.
Four years later, after O'Neal was diagnosed with leukemia, Fawcett returned to his side to offer what help she could.
When Fawcett was diagnosed with anal cancer in 2006, the couple moved in together. Constantly at her side, O'Neal has traveled with her to Germany for more aggressive treatments and, in recent months, often been Fawcett's voice to the media. He has made it clear that he will continue to be a constant and steadfast fixture in Fawcett's life.
CLICK HERE to read more about Farrah Fawcett's journey.