The Charity Commission says documents provided by Oxfam suggest that the charity may not have "fully and frankly disclosed material details about the allegations at the time."
The investigation comes after the resignation of Oxfam Great Britain's deputy chief executive, who apologized to the government, donors and the people of Haiti amid reports that Oxfam staff paid for sex while working among people devastated by a 2010 earthquake.
David Holdsworth, the commission's deputy chief executive, says "issues revealed in recent days are shocking and unacceptable. It is important that we take this urgent step to ensure that these matters can be dealt with fully and robustly.