Speaking to the Today show in a two-part interview set to air tomorrow and Thursday, O'Neal said that while the 62-year-old had mostly remained "so happy" during her battle, it was just last year that things took a significant turn for the worse.
"It's insidious, cancer," he said. "It thinks—it lets you think you're ahead.
"She was athletic and healthy and hungry and beautiful. Everything was just as you remember about her."
That is, O'Neal said, until "five or six months ago."
"The first time I ever noticed, I walked with her and Redmond on the beach one day. We used to take that walk all the time to the rocks and back. And halfway there she stopped and said, 'Can we go back?'
"And Red and I looked at each other and we knew that...See, she could fool you because she always looked good. She looked really good. And he used to say to me, 'She'll be all right. Look how nice she looks.' "
And while Fawcett is currently bedridden and believed to be near the final stages of her cancer battle, O'Neal said she's "not afraid" of what may come.
"I never saw fear at all. She asked me once, 'Am I gonna make it?' She asked me that a couple of weeks ago.
"I said, 'Yeah, sure you'll make it. And if you don't, I'll go with you.' And she said, 'Then stop the Gleevec.' "
Gleevec is the name of the medication O'Neal takes to treat his leukemia.
"So she made a joke," he said. "'Stop the Gleevec.' "
As for how his once tumultuous relationship with the erstwhile Charlie's Angel has changed in the wake of her medical troubles, O'Neal said their bond has grown stronger than ever.
"I know this, that in the last two years I loved her more than I've ever loved her—ever.
"She's so much more of a woman and a powerful, courageous, fearless...all those adjectives. And I look at her with awe."
Officers escorted Redmond to the family's Malibu home, keeping him in "leg [irons], restraints, everything. Like he was John Dillinger. And then they unfastened him and let him go back and see his mother, who was in bed.
"When he went in to her, I said, 'Don't rattle your chains,' because he had them on his legs."
As for his son's recent legal woes, O'Neal said, "He's in jail so much. Poor jerk. That's almost ridiculous."
O'Neal went on to say that while the media has recently taken to referring to him as the rock of his relationship with Fawcett, he claims it's actually the other way around.
"She's the rock. She taught us all how to cope. She's extraordinary. I don't know what I'll do without her, to tell you the truth."
Farrah's Story airs this Friday on NBC.