The appointment follows a pitch against its fellow incumbent agencies, ZenithOptimedia and Carat.
EasyJet appointed OMD to the offline account in May. OMD had pitched against Carat, ZenithOptimedia and MediaCom.
OMD will now handle online planning and buying for easyJet across European markets including the UK, France, Germany, Italy, The Netherlands, Spain and Switzerland. ZenithOptimedia previously handled the UK online task.
The pitch was overseen by David Meliveo, the marketing services manager at easyJet. Leigh Terry, the head of online at OMD UK, will run the business.
Meliveo said: "Throughout the online pitch process, OMD Digital proved to have the strongest team and a clear understanding of easyJet's strategic needs. We believe that OMD will rise to the challenge of integrating our on- and offline business to produce highly effective and innovative campaigns."
Steve Williams, the OMD UK managing director, said: "It is a huge compliment to be awarded easyJet's online business, adding to our offline success recently, up against stiff competition in Zenith-Optimedia and Carat."
The aim of online advertising is to drive traffic to the easyjet.com website.