The world's most famous cyclist confessed to doping during an interview with the US TV host, just a couple of hours after a wrenching apology to staff at the Livestrong charity he founded and has now been forced to surrender.
The emotional day ended with two and a half hours of questions from Oprah at a downtown Austin hotel, where she said the world's most famous cyclist was "forthcoming" as she asked him in detail about doping allegations that followed him throughout his seven Tour de France victories.
Speaking on CBS This Morning, Oprah said she hadn't planned to address Armstrong's confession before the interview aired on her OWN network but, "by the time I left Austin and landed in Chicago, you all had already confirmed it".
"So I'm sitting here now because it's already been confirmed," she added.
The session was to be broadcast on Thursday but Oprah said it will now run in two parts over two nights because there is so much material.
The TV host would not characterise whether Armstrong appeared contrite but said he seemed ready for the interview. "I would say he met the moment," she said.
The confession was a stunning reversal for a proud athlete and celebrity who sought lavish praise in the court of public opinion and used courtrooms to punish his critics.
For more than a decade, Armstrong dared anybody who challenged his version of events to prove it. Finally, he told the tale himself after promising over the weekend to answer Winfrey's questions "directly, honestly and candidly".
The cyclist was stripped of his Tour titles, lost most of his endorsements and was forced to leave Livestrong last year after the US Anti-Doping Agency issued a damning, 1,000-page report that accused him of masterminding a long-running doping scheme.