She will relinquish her position as deputy director of PR and sponsorship in February.
The government marcoms body is on the hunt for a head of corporate communications to take on O'Reilly's responsibilities, reporting jointly to director of PR and sponsorship Oliver Hickson and COI chief executive Alan Bishop.
O'Reilly is relocating to Yorkshire, where she hopes to pursue her career, remaining in the industry.
During her spell at the COI she has been involved in two roster reviews. She said the COI's work has been ‘very much about putting the citizen at the heart of communications', adding that there has been a ‘move in the direction of seeing the public as consumers [of services]'.
O'Reilly this year handled the COI's 60th anniversary, during which it aimed to reinforce its importance by showcasing public-information films from throughout its history.
She joined the organisation at the beginning of 2001 as PR manager, and was promoted first to head of corporate affairs and then to her present role.
Before COI, O'Reilly worked at Citigate Westminster and Beattie Media in Glasgow.
Bishop lauded O'Reilly's ‘magnificent combination of totally serious dedication and a terrific sense of humour needed in the job'.