Mark Goldring said that the scandal had led to 7,000 people cancelling regular donations to the development charity while corporate sponsors appeared to be reserving judgement on whether to pull their support.
Britain's International Development Secretary Penny Mordaunt said that Oxfam GB's then-chief executive, Barbara Stocking, and programme director Penny Lawrence, at the time misled regulators and the charity's donors over the case, "quite possibly deliberately".
Mr Goldring also apologised for remarks made about murdering babies in cots which he made in defending Oxfam.
"I wholeheartedly apologise for those comments and commit to work in that greater public interest so that Oxfam can make a powerful role in the world that we all believe in", he said. We murdered babies in their cots?. Goldring told the Commons worldwide development committee on Tuesday that 7,000 people had cancelled their donations since the scandal broke.
His comments were echoed by Winnie Byanyima, Executive Director of Oxfam International and Oxfam chair Caroline Thomson, who both said they were "ashamed" by the scandal of abuse.
In Scotland four charities have issued statements relating to misconduct probes following the Oxfam scandal.
The threats and intimidation were among several accusations of misconduct made public for the first time with Oxfam's release of its 2011 report into the conduct of its Haiti staff, including the hiring of prostitutes.
Oxfam and Save the Children bosses will go before MPs this morning following allegations of abuse and misconduct by staff working after the natural disaster in Haiti.
"The hard thing to know in these circumstances is whether you're catching the tip of the iceberg or whether you're catching the iceberg itself", Watkins said. We now know that was not enough.
"We've taken lots of measures to improve internal safeguarding measures", he said following a more than two-hour meeting with Haiti's minister of planning and external cooperation, Aviol Fleurant, who had summoned the charity to explain itself.
"We will not work with anybody who does not meet the high standards that we set", May added during a visit to a London school. "So this is painful for me".
Global development secretary Penny Mordaunt has threatened to axe government cash amid allegations some of Oxfam's aid workers used prostitutes in Haiti following the 2010 quake.
Oxfam has pledged to overhaul its recruitment and vetting procedures and submit to an independent commission to investigate allegations of sex abuse.
A source, who was aware of the investigation and was in Haiti at the time, told the BBC that drivers were forced to deliver prostitutes to Oxfam villas.