Blue Rubicon had worked on the account since 2009 but did not repitch, instead handing responsibility to sister agency Surname & Surname.
PrettyGreen will be delivering integrated press office and campaign support, starting with the launch of Giffgaff’s new campaign ‘We’re all the boss’.
The appointment was made after a pitch through matchmaker AAR. O2 reviewed its needs as it sought to expand beyond its traditional heartland of SIM-only deals using the O2 network, starting to sell a selection of 20 handsets last October.
PrettyGreen will report to Giffgaff brand director Tom Rainsford, who said the agency had been selected for its "original approach".
"At Giffgaff we strive to do things a little differently, and the same goes for our agencies," he said.