"This behaviour was totally unacceptable, contrary to our values and the high standards we expect of our staff", Oxfam, one of Britain's largest charities, said in a statement on Friday.
The former director in the country was also accused of inviting prostitutes to his flat rented by Oxfam, according to a leaked report seen by the Times.
The charity watchdog has demanded that Oxfam provide details of allegations that senior aid workers used prostitutes in earthquake-hit Haiti.
The Haiti disaster killed 220,000 people and left millions homeless.
In its statement Friday, Oxfam said that it had informed its trustees, the United Kingdom government's Charity Commission and global development agency, as well as other major donors such as the European Union, the World Health Organization and other United Nations agencies of the investigation and the outcome.
It dismissed four staff members while a further three, including van Hauwermeiren, resigned before the investigation concluded.
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Roland van Hauwermeiren, 68, who Oxfam has confirmed was forced to resign as Haiti country director in 2011 after allegedly admitting hiring prostitutes, went on to become head of mission for Action Against Hunger in Bangladesh in 2014, according to The Times.
One source told the paper employees organised a "full-on Caligula orgy" filled with girls dressed in Oxfam T-shirts.
"Allegations such as those involving Oxfam staff risk undermining public trust".
In the first wave of humanitarian aid after the quake, Oxfam helped around 700,000 people in the Caribbean country.
The Times reported on Friday that the government secretary responsible for charity regulation was calling for Oxfam to provide more information about the charity staff who had paid for sex in Haiti.
She said: "We are now engaged with the charity regarding its approach to safeguarding following more recent allegations".
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At the time the organisation wrapped its investigation, it issued a press release that did not mention sexual misconduct, the Times reported, saying that some staff had been involved "in a number of instances of misconduct", the Times reported.
An Oxfam spokesman said: 'This was not a cover-up. Oxfam's code of conduct bans paying for sex.
'The investigation and its outcome were publicly announced and we kept Oxfam Trustees, the Charity Commission and the DFID informed.
Stocking was chief executive of Oxfam when the allegations emerged in 2011, during which she said, "Oxfam never tolerates misconduct by our aid workers".
'The country director took full responsibility for events that took place under his management and was permitted to resign on the basis that he fully cooperated with and supported the investigation.
The UK's Department for International Development (DFID) said Oxfam's leaders "showed a lack of judgment" in the way they investigated the case and in communicating with the government and the British commission that control humanitarian organizations.
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