Oxfam said the decision to employ the man as a consultant in the African nation was 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 earthquake-hit Haiti.
In a statement, Oxfam said re-hiring one of those workers in Ethiopia “should never have happened”.
“We are still checking how this occurred,” the charity added.
Oxfam said it was investigating whether there were “any issues” while the man was posted in Ethiopia.
The charity’s former director in Haiti, who has been at the centre of the scandal, has reportedly responded by hitting out at “many lies and exaggerations” but admitted he does not “deny everything”.
Roland van Hauwermeiren, who is said to have resigned before the end of Oxfam’s investigation into the Haiti allegations, spoke out in an interview with local media in his native Belgium.
“What I see being published everywhere, is hard to bear. It hurts,” the 68-year-old told newspaper De Standaard.
“But you should know that a lot of people, including in the international media, will blush with shame when they hear my version of the facts.
“It is not that I deny everything. There are things which have been described correctly. But there are many lies and exaggerations.
“Parties every week? Fancy villas? Women paid with money from the organisation?”
Oxfam has come under fire after it was accused of failing to pass on information about Mr van Hauwermeiren’s resignation to a French charity, which later employed him in Bangladesh.
Oxfam has since faced further allegations that prostitutes were also used by its staff in Chad in 2006, where Mr van Hauwermeiren was the charity’s country chief at the time.
Archbishop Desmond Tutu is the latest high-profile figure to quit his Oxfam role over the scandal.
The retired South African church leader said he was stepping down as a global ambassador for the charity, saying he was “deeply disappointed by allegations of immorality and possible criminality” by some Oxfam staff.
It comes after actress Minnie Driver resigned as an Oxfam ambassador, saying she was “horrified” by the sex claims.
Meanwhile, film producer Livia Firth – the wife of British actor Colin – said she was “appalled” by the scandal and was no longer an active ambassador.
More from Oxfam
Archbishop Desmond Tutu quits Oxfam role over charity’s sex scandal
Oxfam’s shamed ex-Haiti chief Roland van Hauwermeiren hits back at ‘lies’
Oxfam ambassador and film producer Livia Firth rails against sex scandal
‘Oxfam’s reaction to the abuse scandal shows we can’t trust them’
Oxfam scandal: Haiti official was sacked by charity over ‘inappropriate behaviour’
Priti Patel: Oxfam boss Mark Goldring ‘should resign’
Some 1,270 people cancelled their direct debits to Oxfam between Saturday and Monday after the Haiti revelations were first reported – almost double the average of 600 cancellations per month.
Oxfam said that it was “too early to tell the impact of the crisis” on donations.