Leno told Oprah Winfrey on her talk show that he told "a white lie on the air" when he said he was going to retire because he assumed he would find another job in show business.
He said the experience "was pretty shocking".
NBC has now unseated O'Brien from Tonight and moved Leno back to the programme he had hosted for 17 years. The star is scheduled to return to Tonight on March 1.
Leno also said he "felt really bad" for O'Brien.
He told Oprah he thinks what happened to O'Brien is "unfair".
Leno also said he has "regrets" that the situation wasn't handled better - but he isn't sure what he could have done differently.
Harpo Productions has asked O'Brien to appear on The Oprah Winfrey Show, but an appearance hasn't been confirmed.