The Charity Commission said some of Oxfam's failings and shortcomings amounted to mismanagement, prompting an official warning.
There was a "culture of poor behaviour" among Oxfam staff sent to help victims of the 2010 Haiti earthquake, with serious allegations of wrongdoing including sexual abuse of children which were not fully disclosed, an inquiry has found.
The lengthy report from the Charity Commission for England and Wales, published on Tuesday after an 18-month investigation, found the charity failed to heed warnings - including from its own staff - that its culture and response to keeping people safe was inadequate, and that subsequent commitments to improve safeguarding were not backed up by actions.
The findings said Oxfam failed to adequately investigate allegations that children as young as 12 or 13 were victims of sexual misconduct against a charity "boss", that it did not report allegations of child abuse by charity staff in Haiti, and that senior staff implicated in sexual misconduct claims were dealt with more leniently than junior figures.
Charity Commission chief executive Helen Stephenson said: "What went wrong in Haiti did not happen in isolation.
"Our inquiry demonstrates that, over a period of years, Oxfam's internal culture tolerated poor behaviour, and at times lost sight of the values it stands for."
She said "significant further cultural and systemic change" is required at Oxfam - which has been under the leadership of new chief executive Dhananjayan Sriskandarajah since January - to address the failings and weaknesses.
Oxfam was plunged into crisis in February 2018 when it emerged that some of its workers engaged in "sex parties" with prostitutes after the humanitarian disaster in the Caribbean country.
The Charity Commission launched its inquiry amid concerns Oxfam may not have fully and frankly disclosed material details about the allegations at the time in 2011, its handling of the incidents since, and the impact these have had on public trust and confidence.
The report said the incidents in Haiti identified in 2011 were not "one-offs", with evidence of behavioural issues as early as June 2010.
Oxfam's Chair of Trustees, Caroline Thomson, said: "What happened in Haiti was shameful and we are deeply sorry. It was a terrible abuse of power, and an affront to the values that Oxfam holds dear."
"The Commission's findings are very uncomfortable for Oxfam GB but we accept them. We now know that the 2011 investigation and reporting of what happened in Haiti was flawed; more should have been done to establish whether minors were involved.
"The decision to allow the Country Director to resign without a fuller investigation of his own conduct would not be permitted today, under our current policies and practices. And while the Commission makes clear that it found no record of a 'cover-up,' we accept that Oxfam GB should have been fuller and franker in its initial reporting of the allegations."
Oxfam's internal investigation, following allegations by a whistleblower in 2011, identified four staff who either did or were suspected of using prostitutes, including on charity residential premises.
The charity could not conclude whether minors were involved in some of the incidents, something the regulator said should have been looked into more closely.
Separately, two allegations of physical abuse, made by email from a 12-year-old and a 13-year-old girl, were "suspected" not to be genuine by Oxfam at the time.
The Charity Commission said that despite there not being evidence to back up the allegations, Oxfam should have tried harder to substantiate the claims at the time.
The report, which also examines Oxfam's conduct in the years after the Haiti allegations, found Oxfam failed to make sufficient "full and frank disclosures" about allegations against the charity in 2011, but said there was no evidence of a cover-up.