Today's decision ends an intense pitch process first revealed by Media Week three months ago.
OMD's successful team was led by Colin Gottlieb, chief executive of Omnicom Media Group (OMG) for Europe, Middle East, and Africa, and comes despite indications the telecoms giant was in line to appoint WPP's Team Vodafone to handle the business, as reported by Media Week.
The appointment is an enormous blow to Aegis's Carat in the UK, which only won Vodafone's £55m media account in January after a protracted seven-month pitch process against previous incumbent, OMD.
A Vodafone spokesman said: "As already discussed with Media Week, we have decided to consolidate all our global media planning and buying business and have chosen OMD to do this.
"It includes activity in more than 20 markets and represents approximately £800m worth of billings.
"There will obviously be a period of transition in all local markets and it is unclear at this stage when OMD will resume full control."
The spokesman also confirmed the pitch process remained a three-way competition until this week's decision, with no agency being knocked out at an earlier stage.
The pitch process was run by Vodafone's global brand director David Wheldon and global media director Charlie Stopford. It supersedes a number of local-market reviews that have already taken place or that are still under way.
The global review was instigated by Vodafone chief executive Vittorio Colao, who took control of the company in July 2008.