The disgraced cyclist came clean to the talk show queen in a candid chat due to air on Thursday (17Jan13), Winfrey announced on Tuesday (15Jan13).
Speaking to CBS This Morning, Winfrey calls her grilling of Armstrong ""the biggest interview I've ever done,"" and admits she ""was satisfied"" and ""riveted by some of his answers"" to the 112 questions she prepared.
She also reveals Armstrong, who was stripped of seven Tour de France titles last year (12), was ""forthcoming"" when she asked him about claims he used illicit performance-enhancing drugs throughout his career.
Winfrey had no plans to address Armstrong's stunning admission before the interview aired, but insists, ""By the time I left Austin (after the interview), you all already had confirmed it. So I'm sitting here now because it's already been confirmed.""
She adds, ""(He was) truthful, serious... certainly had prepared himself for this moment. I would say he did not come clean in the manner I expected. It was surprising to me... for myself, my team, all of us in the room.""
The taping reportedly followed Armstrong's appearance at the Livestrong charity offices in Austin, Texas, where he is believed to have apologised to the 100 members of staff.
He stepped down as chairman of the cancer charity last year (12) as the doping scandal began to dominate headlines.
The interview will be broadcast on Winfrey's U.S. network OWN in two parts.