The appointment of OMD comes after a final stage of a pitch process that pitted it against Carat. MediaCom and ZenithOptimedia had also been involved in the earlier pitch stages.
OMD takes over the consolidated £21 million UK media account for easyJet as well as media in other European markets including France, Italy, Spain and The Netherlands. The business was consolidated after easyJet bought the rival budget flight operator Go last year for £374 million.
EasyJet's £12 million media account was previously handled by Carat's sister agency, BBJ, and ZenithOptimedia had Go's £9 million UK account but was forced out of the pitch process because of increasing conflict with its British Airways client. Feather Brooksbank also handled some work for easyJet.
The media pitch process was overseen by David Magliano, the director of strategic marketing at easyJet.
OMD's appointment was confirmed this week after several weeks of negotiations.
The agency's pitch was led from its London offices.
Alistair Buckle, easyJet's marketing manager, said: "As Europe's number-one low-cost airline, easyJet deserves world-class planning and buying and we believe that OMD can provide this across all European markets."
Colin Gottlieb, the chief executive of OMD Europe, said: "EasyJet is one of the leading players in one of the most competitive markets in Europe. It is a very energetic company that knows its business inside out and to be appointed across Europe is a great compliment for our network."
The appointment of OMD coincides with reports this week that David Beckham's management company, SFX, is threatening legal action against easyJet over an ad campaign that mocks the England captain's latest hairstyle. SFX has reportedly written to easyJet about unauthorised use of his image and asked that the airline make a payment of £10,000 to the NSPCC.