"She came right to my side, which I loved her for. And we gradually started to rebuild our relationship," he told Walters.
As Fawcett helped O'Neal to heal, in 2006, she was struck by the devastating death of her mother and diagnosed with anal cancer -- a relatively rare disease that only affects about 5,000 Americans a year.
"I panicked. I didn't let her know, but I panicked," O'Neal told Walters. "I've been living with cancer for eight years at this point and ... I saw lots of what cancer can do. And I just knew one thing, that Farrah Fawcett was hard to kill."
"Farrah had symptoms for only a fairly brief time before her cancer was diagnosed," said Dr. Lawrence Piro, president of The Angeles Clinic and Research Institute, who began treating Fawcett after her cancer did not respond to the first course of treatment. "So there really wasn't an opportunity to find it earlier, it unfortunately just progressed."
Piro told Walters that Fawcett's cancer was treatable, but not curable.
"We had to use the best tools that we could to try to suppress the tumor, but that we would never get rid of it. So, eventually, the likelihood is that she would succumb to her tumor," he said.
With the 2006 diagnosis, Fawcett and O'Neal moved in together.
At her side for the past three years, O'Neal traveled with Fawcett to Germany for more aggressive treatments and, in recent months, was often Fawcett's voice to the media, making it clear that he will always be a constant and steadfast fixture in her life.
O'Neal told Walters that Fawcett is the only woman he's ever really loved.
"He loves her so much," Farrah's longtime friend Alana Stewart told Walters. "When he walks in the room, her face just lights up."
Watch a special edition of "20/20" TONIGHT at 10 p.m. ET for the full Barbara Walters special "Farrah's Love."