The admission came in an interview to be broadcast on Thursday on Winfrey's network.
A source tweeted: "Just wrapped with (at)lancearmstrong More than 2 1/2 hours. He came READY!"
Armstrong's admission came hours after an emotional apology to the Livestrong charity that he founded and turned into a global institution on the strength of his celebrity as a cancer survivor.
Oprah's interview with Armstrong can be seen on OWN beginning at 2am Friday GMT, and on Oprah.com.
The confession was a stunning U-turn for Armstrong ( 41) after years of interviews and court battles in which he denied doping and zealously protected his reputation.
The cyclist was stripped of his Tour de France titles, lost most of his endorsements and was forced to leave his foundation last year after the US Anti-Doping Agency (USADA) issued a damning, 1,000-page report that accused him of masterminding a long-running doping scheme.
About 100 staff members of the charity Armstrong founded in 1997 gathered in a conference room in Austin, Texas, as Armstrong arrived with a simple message: "I'm sorry."
He choked up during a 20-minute talk, expressing regret for the long-running controversy over performance-enhancers he had caused, but stopped short of admitting he used them.
Before he was done, several members were in tears when he urged them to continue the charity's mission of helping cancer patients and their families.
"Heartfelt and sincere" is how Livestrong spokesman Katherine McLane described his speech.
Armstrong later huddled with almost a dozen people before stepping into a room set up at a hotel.
No further details about the interview were available because of confidentiality agreements signed by both camps.
But Winfrey promoted it as a 'no-holds barred' session, and after the USADA report she had plenty of material for questions.
USADA chief executive Travis Tygart, a long-time critic of Armstrong, called the drug regimen practised while Armstrong led the US Postal Service team "the most sophisticated, professionalised and successful doping programme that sport has ever seen".
Armstrong also went after his critics ruthlessly during his reign as cycling champion, scolding some in public and waging legal battles against others in court.
At least one of his opponents, the UK's Sunday Times, has already filed a lawsuit to recover about $500,000 (¤370,000) it paid him to settle a libel action.
The lawsuit most likely to be influenced by an Armstrong confession might be the Sunday Times case. Potential perjury charges stemming from his sworn testimony in the 2005 arbitration fight would not apply because of the statute of limitations. Armstrong was not deposed during the US government investigation that was closed last year.
Armstrong is said to be worth around $100m (¤75m), but most sponsors dropped him after USADA's scathing report and soon afterwards he left the board of Livestrong.
After the USADA findings, he was also barred from competing in triathlon or running events. World Anti-Doping Association (WADA) rules state his lifetime ban cannot be reduced to less than eight years.
WADA and USADA officials could agree to reduce the ban further depending on what information he provides and his level of co-operation.
Whether his confession would begin to heal those ruptures and restore that reputation remains to be seen.
Diagnosed with testicular cancer in October 1996, the disease spread to his lungs and brain. Armstrong's doctors gave him a 40pc chance of survival and never expected he would compete at anything more strenuous than gin rummy.