Former Deputy Chief Executive Penny Lawrence also resigned on Monday as the scandal continued to grow.
The charity faces a potential funding crisis as the UK Government reviews its relationship with the organisation in light of what worldwide development secretary Penny Mordaunt has described as "deeply despicable" allegations.
The British Government has now threatened to cut aid funding to charities that fall short on safeguarding and do not cooperate with authorities investigating alleged sexual abuse.
The British Government said Sunday it is warning all charities that receive United Kingdom aid to step up efforts to tackle sexual misconduct among staff or face having their funding cut, amid further fallout from a prostitution scandal involving Oxfam workers in Haiti in 2011.
Three other staff members, including the country director Roland van Hauwermeiren, resigned before the end of a 2011 investigation into the Haiti incidents.
"The approach that we are taking is that we need to triple check that matters of the past have been appropriately dealt with", she said.
Documents provided by Oxfam have led to further questions and suggest that the charity might not have "fully and frankly disclosed material details about the allegations at the time", the Charity Commission said. "It is now clear that these allegations - involving the use of prostitutes and which related to behavior of both the Country Director and members of his team in Chad - were raised before he moved to Haiti", Penny said.
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Previous revelations spurred the United Nations in 2002 to issue special measures for all its staff and others, including aid workers under UN contract, based on a policy of zero tolerance.
Britain will stop funding overseas aid agencies if they fail to learn the lessons from Oxfam's sex abuse scandal, and the government will discuss possible prosecutions with law enforcement, the British development minister said on Wednesday.
Oxfam received £31.7 million (35.7 million euros, $43.8 million) from state coffers past year. It's unclear what role the former Oxfam employee was given at the Catholic charity.
She said the crackdown should be a wake-up call to the aid sector.
The report also highlighted instances of rape, verbal sexual abuse, child pornography and prostitution and trafficking of youngsters, many poor, displaced or orphaned by conflict.
"I would like to place on record my honest thanks for the years of dedicated service that Penny has given to Oxfam and the fight against poverty around the world".
"And unless you report every serious incident or allegation, no matter how damaging to your reputation, we can not be partners".