© BBC / Whizz Kid
The children's TV host-turned-comedian will perform in Saturday's (March 3) edition of the charity dance contest.
"I'm leg warmered-up for Let's Dance for Sport Relief, having not done any exercise for a year since being pregnant and having my son, so I hope I've got the stamina for a whole two minutes," she jested.
"I mucked up my wedding dance and that was only a minute long, so concentration will be key for me."
O'Brien will face Omid Djalili, Outnumbered's Tyger Drew-Honey and Dani Harmer, Arabella Weir and the team of Scott Mills and Olly Murs.
"I've no time to think about the competition," she commented. "Although I think Scott and Olly will be a force to be reckoned with."
The Gadget Show pair Suzi Perry and Ortis Deley, comedian Rowland Rivron and Fatima Whitbread are still to compete in Let's Dance.
EastEnders stars Laurie Brett and Tameka Empson, Terry Alderton, Miles Jupp and Eddie 'The Eagle' Edwards have already been voted through to the final.
Let's Dance for Sport Relief continues on Saturday (March 3) at 7pm on BBC One.
