Fawcett died of cancer June 25.
O'Neal, her off-and-on boyfriend for more than 30 years, was by her side when she passed away.
But Fawcett's college boyfriend Greg Lott told TV's "Inside Edition" he was her secret lover for the last 11 years of her life and claimed O'Neal refused to let him see her at the end.
"This guy barred me from seeing the love of my life for the last three months of her life," Lott said in the exclusive interview, adding the actor also kept him from attending Fawcett's funeral.
Lott, now a Texas drug counselor, said he and Fawcett were college sweethearts and dated all through school in the late 1960s, but broke up when he went to prison for selling drugs.
Fawcett moved to Hollywood and landed a starring role on TV's "Charlie's Angels." She also started dating O'Neal around that time.
But in 1998, Fawcett and O'Neal's romance was on the rocks, Lott said, and the actress called him to complain about O'Neal.
"We just got to a point by 1998, we didn't need a piece of paper… we had a commitment, a total commitment," he said.
Lott also revealed he often visited Fawcett at her penthouse condo in Los Angeles and they wrote each other intimate love letters, even when Fawcett was in Germany undergoing treatment for cancer.
When Fawcett returned to the United States, she entered a Santa Monica, Calif., hospital under an assumed name, he said.
"She always had a code name, so I found her, got her on the phone… and talked to her for a week… talked to her until the Thursday night before Easter, and she said I'll call you tomorrow night… I never talked to her again," Lott told "Inside Edition."