The 16-page story, 'Happy Birthday, Dear Happy', will be included in a direct mailing created by WWAV Rapp Collins London.
The mailing and a covering letter from O'Hagan explaining why he supports Unicef is being sent to 53,000 literature lovers on October 20. An additional 150,000 copies of the story, bound in an illustrated booklet, are being given out free with purchases at Waterstones, along with a fold-out slip inviting readers to support the charity.
O'Hagan, who was long-listed for the 2006 Booker Prize, is the children's charity's UK ambassador for literature. The short story for Unicef was inspired by the author's visit to Malawi to see first-hand the work done by Unicef.
Vicky Searle, Unicef's head of direct marketing, said: "Our Unite for Children, Unite against Aids campaign is one-year old on October 25. Andrew's kind contribution of his short story enables us to engage potential donors with a complex, difficult message by reading a beautifully crafted piece of literature."
O'Hagan's first novel, 'Our Fathers', which was published in 1999, was shortlisted for the Booker Prize, the Shortbread First Novel Award, the International Dublin IMPAC Award and the John Llewellyn Rhys Prize. It won the Holtby Prize for Fiction.
His second novel, 'Personality', was published in 2003. He won the EM Forster Award from the American Academy of Arts and Letters and was named one of Granta's Best of Young British Novelists.
His latest novel, 'Be Near Me', was published in August.
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