The entertainment publicity agency will advise on existing and new VIP relationships and assist the in-house PR team with media relations, lending its expertise in dealing with consumer magazines and luxury brands.
It will consult on projects including the Oxjam music festival, other festival events and fashion initiatives.
The agency, which has an existing relationship with Oxfam, has been awarded a one-year contract and is not taking over from another agency.
Liz Matthews PR has also done similar work for the British Heart Foundation.
Oxfam recently fell out with former global brand ambassador and Hollywood actress Scarlett Johansson, who resigned from her role at the charity after signing up to appear in advertising for Sodastream. Oxfam is opposed to the company having a factory in an Israeli settlement in the West Bank.