The anti-poverty organization has been reeling since the Times of London reported last week that seven former Oxfam staff members who worked in Haiti faced misconduct allegations that included using prostitutes and downloading pornography.
Oxfam's Deputy Chief Executive Penny Lawrence resigned on Monday over the handling of the scandal and the United Kingdom government threatened to cut funding to the charity's projects.
The actress added later on Twitter that she was "devastated by the response" of Oxfam which she had been "raising awareness for since I was nine years old".
She said that one of the allegations involved an adult and child volunteer in the United Kingdom, adding: "That troubled me because I knew that Oxfam was not conducting the criminal record checks that it needed to conduct".
The scandal has led to the resignation of Oxfam's deputy head and has thrown into question British government funding for the charity, which amounted to around £32 million (36 million euros, $44 million) previous year.
Britain's secretary for global development, Penny Mordaunt, said Sunday that the way allegations of sexual misconduct were handled by Oxfam was "shocking" and demonstrated an "absolute absence of leadership".
Bill Haas injured in fatal Southern California auto crash
The 2011 FedEx Cup champion was set to return to action on Thursday in the Genesis Open at L.A.'s Riviera Country Club. Haas, 35, was in a Ferrari that collided with a BMW. "His vehicle basically just got clipped by the Ferrari", he said.
Mordaunt called allegations about the use of prostitutes by some Oxfam staff in Haiti in 2011 "horrifying".
Britain's Charity Commission has now announced it has opened a statutory inquiry into Oxfam and International Development Secretary Penny Mordaunt is due to give a speech on Wednesday threatening to cut government funding to charities who fail to put robust safeguarding measures in place.
Both Oxfam and Save the Children Australia say they are urgently reviewing their staff-vetting and monitoring procedures in light of the developments.
The investigation was launched a day after Oxfam Great Britain's deputy chief executive resigned and apologised to the government and donors. "The case for the type of work done by Oxfam is too strong to allow it to be undermined by bad behaviour", he said.
Following news that Oxfam staff were accused of sexual misconduct, including allegations they paid vulnerable women for sex in Haiti after the quake in 2010, Ms Miller said: 'Oh my God, he's been doing this for years'.
Her resignation came as:- The UK's biggest firms, including M&S and Visa, reportedly said they could axe donations to the charity.
She also said her department had created a new safeguarding unit to urgently look into how we can stop sexual abusers and predators being reemployed by charities. This epidemic is rooted in the unequal power relationships that enable powerful and predatory men to exploit women and children through bullying, sexual harassment and outright violence.