Helen Evans said she was told about two women being coerced to have sex.
The ex-head of global safeguarding also heard of a worker who had failed to say he had previously being struck off for sex abuse.
It came as the Oxford-based charity was given until the end of the week to detail how it will handle any further allegations.
Ms Evans told Channel 4 News: "There was... a woman being coerced to have sex in a humanitarian response by another aid worker, another case where a woman had been coerced in exchange for aid and another one where it had come to our attention where a member of staff had been struck off for sexual abuse and hadn't disclosed that, and we were then concerned about what he might be doing, and that was three allegations in one day."
An inquiry into Oxfam's sex scandal by the Charity Commission is due to get under way today (13/02) after the resignation of one of the aid agency's most senior figures.
Deputy chief executive Penny Lawrence said she took full responsibility for what had happened on her watch and was sorry for the "harm and distress" it had caused supporters.
PRESS RELEASE: Oxfam announces resignation of Deputy Chief Executive https://t.co/s7wNu70B60— Oxfam News Team (@oxfamgbpress) February 12, 2018
The charity faces a battle to "rebuild the public trust" following crisis talks with the Government over future funding.
Ms Evans, who was head of safeguarding at Oxfam from 2012 to 2015, also said she raised fears that the charity was failing to report incidents and that the number of cases "was likely to be far wider than was being reported".
In a lengthy statement on Twitter, Ms Evans claimed the allegations surrounding Oxfam's work in Haiti were not an "isolated incident" and claims began to "flood in" as reporting mechanisms were strengthened.
In response to Ms Evans' claims, an Oxfam spokesperson said: "We regret that we did not act on Helen's concerns much quicker and with more resources."
The spokesperson added the charity had "introduced a whole range of measures to improve how we deal with safeguarding issues".
Ms Evans also hit out at the Charity Commission, the Government and the Children's Commissioner over the claims, alleging she raised her concerns with all of them in 2015 but no action was taken. Sky News has contacted them for comment.
Oxfam has been accused of covering up the use of prostitutes by staff in Haiti in the wake of the deadly earthquake in 2010. Allegations that prostitutes were used by staff in Chad in 2006 have also emerged.
It has denied a cover-up, but the charity's chief executive has said nine members of staff "behaved in a way that was totally unacceptable".
In a statement released after a meeting with senior Oxfam figures on Monday, International Development Secretary Penny Mordaunt said: "In the 21st century, it is utterly despicable that sexual exploitation and abuse continues to exist in the aid sector."
She revealed that Oxfam "made a full and unqualified apology" to her, the British public and the people of Haiti for the "appalling behaviour".
The scandal has put Oxfam's funding from the Government under threat, with the charity receiving £31.7m in 2016/17.
Ms Mordaunt's predecessor, Priti Patel, wrote in a newspaper article on Monday that the Oxfam claims were "the tip of the iceberg".
The International Development Secretary said charities in general needed to "step up and do more" in the wake of the Oxfam allegations.
As part of a series of measures, Ms Mordaunt:
- Wrote to all UK charities operating overseas, demanding they "step up and do more" so "we have absolute assurance that the moral leadership, the systems, the culture and the transparency that are needed to fully protect vulnerable people are in place"
- Asked such charities to confirm they have referred any cases or concerns about individuals to the relevant authorities
- Established a new unit to "urgently review" safeguarding across the sector to ensure people are being protected from sexual exploitation and abuse
- Pledged to step up work to tackle the problem at the UN and other international bodies
- Announced the department will co-host a summit on the issue by the end of the month along with the Charity Commission, and involving charities that operate overseas