TV
The 74-year-old radio DJ will take over from Des Lynam, who decided to step down from the post after just 18 months.
Co-star Carol Vorderman, 45, told the BBC that O'Connor had so much energy that "he leaves the rest of us exhausted".
Who would fill the presenter role as come under much speculation recently with the likes of Eamonn Holmes, Stephen Fry and Michael Aspel rumoured to be the next host.
Countdown was hosted by Richard Whiteley for 23 years until his death in 2005.
More: TV
Tube Talk
'Freaks and Geeks': Tube Talk GoldTube Talk Gold heads back to the '80s and remembers Freaks and Geeks.
TV Interviews
'Celebrity Apprentice' Debbie Gibson Q&ADebbie Gibson opens up about her Celebrity Apprentice experience.
Reality Bites
Does Simon Cowell have new SuBo?: VideoWatch a clip of Jonathan Antoine, who is tipped for success on BGT.
TV Ratings
Judi Dench draws 4.1m on 'Graham Norton'ITV1 pips BBC One to the highest primetime audience share.
