One of the Oxfam workers sacked over sexual misconduct allegations in earthquake-hit Haiti was later rehired by the charity in Ethiopia.
The decision to employ the man as a consultant in the African nation was described by the UK-based 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.