The announcement solves a delicate problem for NBC, which realized O'Brien was getting antsy in his Late Night time slot (12:35 a.m., Channels 5, 2) and wanted to keep him at NBC.

Leno planned to make the announcement on Tonight Monday, a special celebration of the talk show's 50th anniversary.

"In 2009, I'll be 59 years old and will have had this dream job for 17 years," Leno said. "When I signed my new contract, I felt that the timing was right to plan for my successor, and there is no one more qualified than Conan."

Leno took over from Johnny Carson in 1992 and after a few years of trailing the man he beat out for the job in the ratings - David Letterman - he passed the CBS star and has been dominant in the time slot.

An unknown at the time, O'Brien had the thankless job of following Letterman on Late Night, and he was nearly fired after several weeks. But he recovered and has been a critical and commercial success.

But O'Brien has openly talked about wanting to move on and, in the late-night world, that means an earlier time slot.

The move comes one week after CBS began a rotating series of guest hosts for O'Brien's rival The Late Late Show, whose host Craig Kilborn recently departed.

Guest hosts include Michael Ian Black (Ed), D.L. Hughley (The Hughleys), Amy Sedaris (Strangers with Candy) and sports talk host Jim Rome. There had been speculation when Kilborn quit that Letterman would try to lure O'Brien from NBC with the promise of eventually taking over The Late Show.