RKCR/Y&R will now work on advertising duties for the charity's UK campaigns, and to support its development and relief work around the globe.
Cathy Ferrier, Oxfam's fundraising and supporter marketing director, said: "We were extremely impressed with the agency's response to the brief and their commitment to Oxfam's mission."
Richard Exon, the chief executive of RKCR/Y&R, added: "The scale and ambition of Oxfam's cause is awe-inspiring and we are proud and excited to be moving forward as Oxfam's partner."
The appointment ends a period of substantial change in the charity's marketing arrangements. In August, it appointed ZenithOptimedia to its media planning and buying account following a pitch against Feathers Brooksbank, the 7stars, Profero, Cheeze and All Response Media.
In July, it appointed Leo Burnett to handle advertising duties for Oxfam Unwrapped, its gifting service.