Imagination will be responsible for implementing a fully integrated marketing communications strategy for the alliance.
The account will be predominantly handled by Imagination's New York office, with support from the agency's other nine offices around the world, including Hong Kong and London.
Oneworld brings together 10 of the world's biggest brands in the airline industry including American Airlines, British Airways, Cathay Pacific, Finnair, Iberia, Japan Airlines and Qantas.
Oneworld gives its member airlines benefits they can pass on to their passengers, such as a broader route network, opportunities to earn and redeem frequent flyer miles and more airport lounges.
Filip Lemmens, commercial director of the Oneworld alliance, said: "Imagination emerged as the best agency for our needs out of a very strong shortlist of global agencies.
"Their fresh approach, extensive credentials and remarkable creativity made them a clear choice for Oneworld."