It comes as one of Britain's biggest charities could have its government funding cut over its handling of the row involving its staff.
Oxfam's deputy chief executive has stepped down amid allegations that its staff used prostitutes in Haiti after its devastating earthquake in 2010.
Penny Lawrence, who has worked with the charity for almost 12 years, said she was "desperately sorry for the harm and distress" caused by the controversy.
Her statement read: “Over the last few days we have become aware that concerns were raised about the behaviour of staff in Chad as well as Haiti that we failed to adequately act upon.
"It is now clear that these allegations - involving the use of prostitutes and which related to behaviour of both the Country Director and members of his team in Chad - were raised before he moved to Haiti.
“As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility."
Oxfam funds face scrutiny
Britain's aid minister Penny Mordaunt is also meeting Oxfam chiefs today to discuss preserving its government funding.
Oxfam's executive director Winnie Byanyima says changes have already been made to handle and prevent a repeat of sex abuse claims.
"What happened in Haiti would not pass our rules and our systems today. But we still have more to do and I'm determined that we're going to improve and never will we allow this to happen as leaders in the organization," said Byanyima.
Oxfam, one of Britain's biggest charities, has condemned the behavior of some former staff in Haiti after a newspaper report said aid workers paid for sex while on a mission to help those affected.
The UK aid minister has describing reports of sexual exploitation in the sector as "utterly despicable."