The agency pitched against the incumbent, Total Media, and Rocket to win the work.
OMD worked on two briefs for the pitch: one for Tony Parsons' new hardback The Family Way and the other for the paperback release of My Heart is my Own, a biography of Mary Queen of Scots by John Guy, in order to secure the business.
The agency will provide media planning and buying to support around 70 books a year across authors including Jonathan Franzen and Nigel Slater.
It will work across Harper- Collins' four divisions: its literary fiction division, Harper-Press; Harper Entertainment for popular culture books such as Lord of the Rings; its mind body spirit division, Thorsons/Elements, and Harper Children.
Amy Nelson-Bennett, the marketing director at HarperCollins, said: "I was hugely impressed by the OMD UK team. It clearly demonstrated not just its impressive planning and buying skills, but also a genuine passion for our business and the changing way in which we're looking to market our books and authors. It has a great mix of creativity and commercialism."