Oxfam said the man, who was taken during the hijacking of one of the organisation's vehicles, has now been missing for two months
The organisation said that continued insecurity made it difficult to give humanitarian assistance to the local population.
Paul Smith-Lomas, Oxfam's regional director, said: "More than three million people in Darfur are dependent on humanitarian assistance, and the ongoing insecurity is making it extremely difficult for aid agencies to reach people who are in desperate need."
Oxfam will continue its work at its four other offices in the region.
Smith-Lomas called on world leaders to boost the capabilities of the African Union contingent of peacekeepers, who have so far failed to contain the violence in Darfur.
Khartoum and the largest Darfur rebel group, the Sudanese Liberation Movement, signed a peace deal in May.
But other movements have refused to sign on and factional unrest has since increased.