McCann's London and Oslo offices will lead creative duties on the business, which, until December 2006, were handled by Leo Burnett. Media was formerly handled on a local basis.
Previously, Statoil's advertising has focused on the nine markets in Scandinavia and Eastern Europe, where it runs 2,000 petrol stations. McCann will create a campaign to mark the launch of StatoilHydro in October.
The appointments follow the Norwegian government's approval earlier this month of the merger between Statoil and Norsk Hydro.
The deal, worth EUR30 billion, will create the world's largest offshore oil and gas operator when it is concluded in the autumn.
Kjetil Undhjem, the vice- president of brand management at Statoil, said: "McCann showed us the benefit of global reach without losing the important local touch in our domestic market."