The agency won the business after a pitch against LBi, Kitcatt Nohr Digitas and M/SIX. Oxfam continues to work with 2020 on its digital media account.
Proximity will work with Oxfam to deliver an integrated digital activation strategy to acquire new donors and drive income growth.
It will also work alongside Rainey Kelly Campbell Roalfe/Y&R on an integrated campaign this summer.
Paul Clarke, the director of fundraising and supporter marketing at Oxfam, said: "We are looking to drive innovation in how we engage with our supporters and see digital as a key opportunity."