The Love Story actor, who was in a relationship with Fawcett before she died from anal cancer in 2009, told Piers Morgan on his chatshow that he believes his uneasy relationship with daughter Tatum during that time could have had a negative affect on Fawcett's health.
Ryan said: "What bothers me the most is that there was turmoil during my love affair with Farrah, a lot of it caused by my family and my kids. All of them, but particularly Tatum. And I just think that if she had never met us, would she still be alive today? Because nobody knows what causes cancer."
When Morgan questioned whether he really believed his family stresses could have caused the Charlie's Angels star's cancer, Ryan said: "Maybe it isn't true but it's possible... Sorry Tatum, but you probably know too."
Tatum later asked Morgan on Twitter if he "believed" what her father had said on his show, but received no response. She then wrote: "I am having a hard time swallowing what my dad said tonight on Piers. Wow."
Later, she added: "My dad is both the aggressor and the victim. You can't be both... having lived with this type of person during my formative years took a terrible toll on me and my [brother]."
Morgan described the interview on his personal Twitter page as "the most explosive, emotion-charged, astonishing hour I've ever spent with a celebrity" before the show aired.
Tatum recently revealed that she has to forgive her dad constantly for the "stupid stuff" he does, but said that she has "a boundary" now.
Ryan has previously said that he thinks Tatum is a "bitch" for airing their family's "laundry in the street for money" by writing a book five years ago.
Watch an extract from Ryan O'Neal's appearance on Piers Morgan Tonight below: