The regulator today said its inquiry into the charity had found "a culture of tolerating poor behaviour" and issued it with an official warning.
Responding to the report, Caroline Thomson, who became chair of the charity in 2017, said Oxfam accepted it should have improved its safeguarding practices more quickly.
"What happened in Haiti was shameful and we are deeply sorry," she said. "It was a terrible abuse of power and an affront to the values that Oxfam holds dear."
She said the commission’s findings were "very uncomfortable" for the charity, but it accepted 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," she said.
Thomson said the decision to allow the former Haiti country director Roland van Hauwermeiren to resign without a fuller investigation would not be permitted under the charity’s current policies.
"While the commission makes clear that it found no record of a cover-up," she said, "we accept that Oxfam GB should have been fuller and franker in its initial reporting of the allegations."
The charity said it accepted 79 detailed recommendations from the report to improve its efforts to tackle sexual abuse and reduce the risk of it occurring, and it had already taken action on many of them.
"We should have acted more decisively to address resourcing concerns raised by our own safeguarding team and been more proactive in our reporting of serious incidents," said Thomson.
"But I am confident that Oxfam GB is changing and that the steps we are taking are putting Oxfam on the right path for the future.
"Both the Charity Commission and the independent review of our safeguarding record since 2011 acknowledge the progress Oxfam GB has made over the past year. The review recognises our appetite to improve, citing better referencing and incident reporting, and improved casework, especially in Oxfam shops, as evidence of a sharper focus."
Thomson said Oxfam would act to change its broader working culture in the wake of the inquiry and had welcomed Danny Sriskandarajah as chief executive to lead this change.
Sriskandarajah, who joined the charity earlier this year, said: "As the incoming chief executive, my first duty has been to ensure that Oxfam learns the lessons of the past, and improves our policies and practices.
"Oxfam has a long and proud history of helping to save lives and alleviate poverty around the world, but we know this vital work relies on the respect and trust of supporters and all those Oxfam serves."