Gurpreet Singh was dismissed for sexual misconduct in 2011 but just months later he was employed by the charity 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.
Complaining of "slander", he said he believed reports were based on allegations by a man he had fired in Liberia for drunkenness and abusing staff.
He told Belgian newspaper De Standaard: "A lot of people, including in the global media, will be blushing with shame when they hear my version of the facts".
"It is not that I deny everything", he told the paper. It is alleged that the organisation's money was used for this objective.
Meanwhile, Oxfam said that it was "too early to tell the impact of the crisis" on donations, but revealed 1,270 people cancelled their direct debits between Saturday and Monday - nearly double the average of 600 cancellations per month, reports the Press Association.
Winter Olympics 2018: American luger Emily Sweeney crashes on final run
Sweeney hit the turn at over 60 miles per hour , trying to use her feet to slow down her descent, but to no avail. Sweeney entered the final run in 14th place, having moved up one spot following her first two runs.
Senior staff have already left the charity.
In further developments, the charity said it also sacked its Haiti country director past year following allegations of "mismanagement" and "inappropriate behaviour".
Also on Thursday, British International Development Secretary Penny Mordaunt and the director of the UK National Crime Agency discussed next steps regarding the Oxfam revelations.
The NCA has been "closely monitoring" allegations and confirmed it has a range of powers to investigate sexual offences committed outside the United Kingdom and support foreign security forces.
Andrew MacLeod, a former aid worker and former chief of the United Nations' Emergency Coordination Centre, has been lobbying for the global aid sector to clean up its act.
In his first response to the allegations of sexual misconduct, Roland Van Hauwermeiren said in an open letter to a broadcaster in his native Belgium he did not want to cast himself as a victim but feared that Oxfam, other charity workers and those they help would suffer from false accusations. The aid industry is a magnet for good people to try and do good work.
Save the Children's chief executive, Kevin Watkins, is due to give evidence on wider sexual exploitation in the aid sector and a Government representative will be present to explain its knowledge of the problem.