Workman, who used to be a consultant at Vodafone, will replace OMD global director Ian Rotherham, who has moved after two years to work on another key part of the agency's business.
The former Vodafone exec wil report to Colin Gottlieb, chief executive of Omnicom Media Group EMEA.
Gottlieb said: "Vodafone represents a huge opportunity for OMD and we are delighted to have a professional of Liz’s calibre join the team."
Vodafone appointed OMD to its global media planning and buying account in August last year, after a pitch against WPP's Team Vodafone group.
As part of the move, OMD regained the telecom giant's UK account which it had lost at the beginning of the year when it was moved into Carat.
Workman said: "Vodafone is a fabulously powerful brand in the most compelling category, and OMD is perfectly placed to capitalise on this and thus change the shape of the media agency as we know it. I am thrilled to be leading the charge at OMD."
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