Oates was the party's director of policy and comms for 15 months from 2007. He joined Bell Pottinger last year in the specially created role of strategic media director.
He is now returning to the Liberal Democrats on 1 November to oversee the party's media strategy for the upcoming general election.
He will join the campaign team as director of general election communications. He will report into the recently promoted CEO Chris Fox, who replaced Oates when he left the party. He will also work alongside campaigns and elections director Hilary Stephenson, who is in charge of the ‘ground war'.
Party leader Nick Clegg's strategic comms advisor, and the former MD of Saatchi, John Sharkey will chair the campaign. MP Andrew Stunell will be vice chair.
Oates said the lure of being involved in a Liberal Democrats' election campaign was too strong to turn down. ‘Nick asked me if I would like to go back. When I left I always regretted I hadn't been there for an election. Peter Bingle [Oates' boss at Bell Pottinger] is a political junkie himself and he completely understands my decision'.
Oates said he intended to stay until two months after the election, which has to be held by the first week in June next year. He said he hadn't discussed the possibility of returning to Bell Pottinger but added: ‘We're intending it to be a very successful election campaign, so opportunities may open up in that regard.'
Bell Pottinger Public Affairs chairman Peter Bingle said: 'I regard Jonny as the best strategic communications professional in the business. He has been a friend and colleague for many years. I understand why the lure of returning to front-line politics was too strong. Our loss is Nick Clegg's gain'.