It's being claimed that staff for the charity in Haiti, and indeed in other countries too, paid vulnerable people for sex.
The actress, who actually has Irish connections with family based in Mullingar, said she was "nothing short of horrified" by the sex scandal to rock the charity.
In a statement explaining why she was stepping down from her Ambassador role, Minnie said: "I certainly will not let the abhorrent mistakes of a troubling organization stop me or anyone else from working with good people in this space to support a population of human beings around the world that needs our help."
Since the allegations first came to light, Oxfam's deputy chief executive, Penny Lawrence, resigned over the handling of complaints.

Minnie Driver becomes the first celebrity to quit Oxfam in protest at sex scandal . https://t.co/yPJfG6u4Xk via @telegraphnews

— Minnie Driver (@driverminnie) February 13, 2018

She wrote on Twitter last night: "All I can tell you about this awful revelation about Oxfam is that I am devastated.
"Devastated for the women who were used by people sent there to help them, devastated by the response of an organization that I have been raising awareness for since I was 9 years old."

All I can tell you about this awful revelation about Oxfam is that I am devastated.Devastated for the women who were used by people sent there to help them, devastated by the response of an organization that I have been raising awareness for since I was 9 years old #oxfamscandal

— Minnie Driver (@driverminnie) February 13, 2018