The UK-based charity has been in the headlines after reports in the Times newspaper claimed senior staff had hired sex workers in Haiti while delivering aid after the 2010 quake which struck the island nation.
Oxfam has denied a cover-up over the use of prostitutes by senior aid workers in Haiti following the country's devastating 2010 quake.
Scandal-hit British charity Oxfam was reeling Tuesday after fresh claims of sexual assault and cover-up in South Sudan, as Haiti's president condemned the behaviour of some of its staff in his country as "undignified and dishonest".
Evans also spoke about the "inappropriate conduct" towards teenage volunteers working in Oxfam's United Kingdom charity shops.
GETTYIt is understood Penny Mordaunt never requested current Oxfam chief executive Mark Goldring resigns
The leadership of Oxfam, where deputy director Penny Lawrence has already resigned, also faces questions about what it did to address sexual misconduct more broadly.
On Twitter, she wrote: "All I can tell you about this terrible revelation about Oxfam is that I am devastated".
He said: "We expect our partner organisations wherever they are to monitor their work closely, and to be open, honest and transparent with us, and the public whenever there are any suggestions of abuse or malpractice".
The charity faces a potential funding crisis as the UK Government reviews its relationship with the organisation in light of what global development secretary Penny Mordaunt has described as "deeply despicable" allegations.
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Oxfam said that with 10,000 NGOs working in Haiti alone in 2011, "it was unfortunately not possible for Oxfam to ensure that those found guilty of sexual misconduct were not re-employed in the sector".
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.
Oxfam now faces a battle to prevent other high-profile ambassadors from stepping down as well as making sure the United Kingdom government does not withdraw the £32 million (Dh163m) of funds it receives.
Roland van Hauwermeiren is the guy who's been disgraced from Oxfam for offering vulnerable Haitian women money for sex when he was supposed to be offering them aid and relief (that's sort of the job of a humanitarian worker).
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Oxfam International Executive Director Winnie Byanyima called the entire revelation as "shocking".
She said: "No organisation is too big, or our work with them too complex, for me to hesitate to remove funding from them if we can not trust them to put the beneficiaries of aid first", she said. "And compels us to take action", she said.
Among the allegations is that the head of Oxfam in Haiti had "wild sex parties" with multiple sex workers.
Mordaunt has previously threatened to withdraw government funding from Oxfam unless it gave the full facts about events in Haiti.
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Founded in 1942, Oxfam is one of Britain's best-known charities.
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