But the disgraced cyclist has now said sorry.
"She is one of these people that I have to apologise to. She's one of these people that got run over, got bullied. I have reached out to her and tried to make those amends on my own," Armstrong said during an interview with Oprah Winfrey.
Asked to confirm if he sued O'Reilly, Armstrong said: "We sued so many people I'm sure we did.
"It's a major flaw. It's a guy who expected to get whatever he wanted and to control every outcome. It's inexcusable."
Armstrong attempted to discredit Ms O'Reilly by calling her a "whore". "I was just on the attack," he said.
Ms O'Reilly had revealed the cyclist received a back-dated therapeutic user exemption certificate for corticosteroids at the 1999 Tour de France to ensure he did not test positive.
Armstrong admitted to being a bully and a control-freak after discrediting witnesses who exposed his drug use.
"Yes, I was a bully. I was a bully in the sense I tried to control the narrative. And if someone challenged that I would simply say 'that's a lie, they are the liars'," he said.
The 41-year-old survived cancer and went on to win seven successive Tours de France.
Another person who came under attack from Armstrong was Betsy Andreu, the wife of Armstrong's former team-mate Frankie Andreu.
But he refused to confirm or deny claims she heard him telling doctors at Indiana University Hospital that he had used steroids.
He was being treated for cancer at the time.
While Armstrong has recently spoken at length, they are yet to make peace.
He said: "She asked me and I asked her not to talk about the details of the call. It was a personal conversation.
"They (the Andreus) have been hurt too badly and a 40-minute conversation isn't enough. I said, 'listen, I called you crazy. I called you a bitch. I called you all these things, but I never called you fat'."
The United States Anti-Doping Agency compiled evidence from 11 former team-mates, O'Reilly and Betsy Andreu, which led to Armstrong's downfall and public confession.