Fawcett, 62, has been battling anal cancer for more than two years.
"It's a love story. I just don't know how to play this one. I won't know this world without her," O'Neal told People magazine exclusively. "Cancer is an insidious enemy."
The actor confirmed the disease has spread to other parts of Fawcett's body, including her liver.
"She stays in bed now," he revealed, adding that her trademark luxurious locks are now gone and she spends most of her days sleeping. "The doctors see that she is comfortable. Farrah is on IVs, but some of that is for nourishment. The treatment has pretty much ended."
Fawcett chronicled her battle with cancer for a two-hour documentary called "Farrah's Story," which is set to air May 15 on NBC.
O'Neal told People that even when her prognosis looked grim and hope started to fade, Fawcett continued fighting.
"There was always a courage there, and a quiet dignity," he said. "Farrah never changed. I fell in love with her all over again because of how she handled this."