One of the Oxfam workers sacked over sexual misconduct allegations in earthquake-shattered Haiti was later re-hired by the charity in Ethiopia.
The decision to employ the man as a consultant in the African nation was described by the charity as a "serious error".
Several staff were sacked or resigned in 2011 over a string of lurid claims, including that they had used prostitutes while delivering aid to Haiti.
Oxfam said in a statement it had identified that "one of those dismissed by Oxfam as a result of the sexual misconduct case in Haiti" was "subsequently hired by Oxfam as a consultant In Ethiopia".
Hiring the man "even in an emergency as a short-term consultant" was a "serious error and should never have happened", it continued.
"We are still checking how this occurred but it further highlights that we need an organisation and sector-wide approach to the vetting and recruitment of both staff and consultants, especially in emergencies where there is pressure to fill posts quickly in order to help save lives. "
The charity is now checking whether there were "any issues" while the man was posted in Ethiopia.