Oxfam officials will try to convince the government it should keep its government funding - despite claims of sexual misconduct by its aid workers.
The charity has been accused of concealing the findings of an inquiry into claims staff used prostitutes while delivering aid in Haiti in 2011.
Oxfam denies a cover-up and officials will meet International Development Secretary Penny Mordaunt later.
The Charity Commission said it should be "frank" about what it revealed.
The charity's own investigation led to four people being sacked and three others resigning, including the country director for Haiti.
Ms Mordaunt has said Oxfam must account for the way it handled the claims or it risks losing government funding, worth £32m in the last financial year.
Michelle Russell, director of investigations at the Charity Commission - which will also be part of the talks - told BBC Radio 4's Today programme officials had been "assured" Oxfam had investigated it "fully".
However, she said the watchdog was not told the full story at the time of the investigation.
"Had the details of what has come out been told to us, we would have dealt with this very differently," she added.
Oxfam said allegations that underage girls may have been involved were unproven.
Andrew MacLeod, a former senior adviser to the UN, who works for Hear Their Cries - a charity that fights sexual exploitation in the aid industry - said the case should be passed on to police.
"If Oxfam were serious... ask Scotland Yard to do an investigation. Were these prostitutes underage or not?" Mr MacLeod told the BBC.
"If they were underage then this man has broken UK sex tourism laws. And Oxfam leaders may have broken the law by aiding and abetting this man."
Oxfam has faced growing criticism of the way it handled the allegations of misconduct by its staff in Haiti, where they were working in the aftermath of the huge earthquake that devastated the country in 2010.
On Sunday, Ms Mordaunt told the BBC's Andrew Marr the charity had failed in its "moral leadership" over the "scandal".
She said Oxfam did "absolutely the wrong thing" by not reporting details of the allegations.
She said no organisation could be a government partner if it did not "have the moral leadership to do the right thing".
'Spur to improvement'
Ahead of the government meeting, Oxfam announced new measures for the prevention and handling of sexual abuse cases.
Oxfam's chairman of trustees, Caroline Thomson, said the charity's board had appointed a consultant earlier this year to review its culture and working practices, which would now be extended.
"It is not sufficient to be appalled by the behaviour of our former staff - we must and will learn from it and use it as a spur to improvement," she said.
The charity will also introduce tougher vetting of staff and mandatory safeguarding training for new recruits.
It said it will also work with the rest of the aid sector to make it easier to share intelligence about people who have been found guilty of sexual misconduct.
This will be in addition to measures designed to prevent sexual abuse and misconduct and improve the handling of allegations which Oxfam says it introduced in the wake of the Haiti case in 2011.
The allegations of misconduct by Oxfam staff in Haiti date from 2011 but came to light in a report in the Times on Friday.
It said the charity's country director for Haiti, Roland van Hauwermeiren, was alleged to have used prostitutes at a villa rented for him by Oxfam in the aftermath of the 2010 earthquake.
Oxfam knew about the allegations at the time and launched an internal investigation, as a result of which four members of staff were dismissed.
Three others, including Mr van Hauwermeiren, were allowed to resign before the end of the investigation.
Mr van Hauwermeiren went on to work elsewhere in the sector, but Oxfam said it would not have provided a positive reference.
The charity says at the time it made public a report, which said "serious misconduct" had taken place in Haiti and issued a press release - but did not give details of the allegations.
It told the Charity Commission it was investigating inappropriate sexual behaviour, bullying, harassment and staff intimidation - but again did not reveal the exact details.
Oxfam's chief executive, Mark Goldring, told the BBC that describing details of the behaviour at the time could have drawn "extreme attention" to it, which he said would have been in no-one's best interest.