Oxfam has revealed that three of the men accused of sexual misconduct in Haiti physically threatened witnesses during a 2011 investigation.
The charity has published its internal report on alleged abuse by some of its staff following public pressure.
In the 2011 report, Oxfam said "more needed" to be done to prevent "problem staff" working for other charities.
Oxfam offered its "humblest apologies" to the Haitian government at a meeting with its minister of planning.
Earlier this month, the Times newspaper published allegations that Oxfam aid workers in Haiti had used prostitutes in 2011. Oxfam denied a cover-up but its handling of the scandal is being investigated by the Charity Commission.
Oxfam - which has almost 10,000 staff working in more than 90 countries - has released a redacted version of its internal report, saying it wants to be "as transparent as possible" about the decisions it made.
Parts of the 11-page document are blacked out to hide people's identities, including the names of the three men accused of intimidating witnesses.
Oxfam presented the original, unedited report to the government in Haiti on Monday.
Speaking after the meeting with Haitian minister Aviol Fleurant, Simon Ticehurst, Oxfam regional director for Latin America, said the charity would be starting "the long road ahead of re-establishing trust and partnership, given our 40-year history with Haiti and its citizens.
"We stand ready to engage with the Haitian people and have expressed our openness to co-operate as much as required with the Haitian government."
Analysis: More revelations are likely
By BBC diplomatic correspondent James Landale
Oxfam is reeling under the revelations that some of its staff were guilty of bullying, intimidation and sexual misconduct, including using prostitutes on the charity's premises.
So it is trying to start recovering some of its reputation by being as transparent as it can. Hence the publication of the internal report into what happened in Haiti in 2011.
In the short term this may produce more damaging revelations not only about the inappropriate behaviour of staff but also the mismanagement and lack of transparency by senior managers.
There are also likely to be more revelations when the Charity Commission and MPs begin their own investigations, as well as Oxfam's own internal inquiry.
So few would disagree with the Prime Minister's spokesman when he says Oxfam has a long way to go to restore public trust.
Theresa May told BBC London the revelation that three Oxfam staff physically intimidated witnesses during the charity's investigation was "absolutely horrific".
"This is exactly the problem that we see which means that all too often people don't feel able to come forward to report what has happened to them," the prime minister said.
One former Oxfam aid worker told the BBC she was sexually and physically abused by a senior male colleague while in Haiti. She claimed she was also sexually assaulted by another Oxfam worker in South Sudan months later.
'Bullying and intimidation'
Seven Oxfam employees left the organisation as a result of their behaviour in Haiti in 2011, the report shows.
One employee was dismissed and three resigned for using prostitutes on Oxfam premises. The use of underage prostitutes was not ruled out.
Two more were dismissed for bullying and intimidation - one of whom also downloaded pornography - and another man was sacked for failing to protect staff.
In the report, the charity said director of operations in Haiti, Roland Van Hauwermeiren, "admitted using prostitutes" at his Oxfam residence when questioned by the investigation team.
Mr Van Hauwermeiren last week denied paying prostitutes for sex.
He was granted a "phased and dignified exit" and was allowed to resign, the report added, on the provision that he fully co-operated with the rest of the investigation.
It is not known if he is one of the suspects accused of threatening witnesses.
The BBC has discovered that one of the staff dismissed from Oxfam for gross misconduct in Haiti was Raphael Mutiku, a Kenyan aid worker who is based outside the capital Nairobi.
Asked by a reporter whether he had been with Mr Van Hauwermeiren, Mr Mutiku said "no". As he got into a vehicle, he was asked why he had been let go by Oxfam but not did not answer and closed its door.
Another section entitled "lessons learned" sets out the need for tighter safeguarding across the industry to stop disgraced aid workers moving to new posts.
It reads: "Need better mechanisms for informing other regions/affiliates/agencies of behavioural issues with staff when they move and to avoid 'recycling' poor performers/problem staff."
Despite the warning, several of those implicated subsequently worked for other aid organisations, including at Oxfam.
Mr Van Hauwermeiren ended up taking another high-profile position, as the head of mission for Action Against Hunger in Bangladesh.
Action Against Hunger said it had carried out a series of checks on Mr Hauwermeiren but received no information from Oxfam of inappropriate or unethical behaviour.
Tory MP Pauline Latham, a member of the Commons International Development Committee, has said there should be a global central aid worker register of suspected perpetrators to stop them moving from one charity to another.
"If they have been abusing the most vulnerable people in the world… we should not be allowing them to go from one charity to another without any stain on their character," she told the BBC's Victoria Derbyshire programme.
A spokeswoman for the Department for International Development said it would not currently be commenting on the report.
On Tuesday, the committee is due to question senior representatives from Oxfam, Save the Children and the Department for International Development on sexual exploitation in the aid sector.