"When you sue people that you know are telling the truth, that is really difficult to forgive," he said.
David Walsh, a British journalist who co-wrote a book that detailed the allegations against Armstrong, said a confession "won't be enough."
"He's got to make reparations to the people he wronged. He's got a lot of apologizing to do," Walsh said.
Walsh's book prominently featured O'Reilly's accounts of doping in the cycling world. Armstrong later sued over the book, including a libel lawsuit against The Sunday Times, which published excerpts. The Times is now suing to recover the settlement it paid Armstrong in the case.
"My feeling is that he came forward because he's been in a pretty bad place since the truth has emerged, and the only way that he can rebuild to begin his life is to make a full confession of all the things he did," Walsh said.
Unlike some of the other critics, Walsh said he didn't feel any vindication at Armstrong's expected about-face. But he said he feels "tremendous satisfaction for the people who helped me," people he said told the truth "at great cost to themselves."
"They were vilified. Their characters were assassinated, and they were in a bad place," he said. "Betsy Andreu has said she spent 13 years telling people she wasn't a liar, and that's a very difficult place to be."
Next month, Walsh will have the catbird seat when he takes part in a university symposium in Leeds, United Kingdom. There he'll discuss his years of reporting on Armstrong, reporting that appears to have finally caught up with the onetime star.
The event is called, simply, "The Pursuit of Lance Armstrong."
Willerton, Armstrong's former teammate, says he's just glad the truth appears to be finally coming out.
"We have Lance Armstrong versus cancer. We had Lance Armstrong versus everyone in the Tour de France. But Lance Armstrong versus the truth? That's one that he really couldn't win in the end."