Hallett joins the high-profile body as director of comms on 12 October after five years with the national organ donation agency.
She will lead a team of 80 - comprising 21 people in comms strategy and brand development, ten staff in publishing, three in design, ten in press, ten in e-comms and 24 in internal comms. She replaces Helen Rafalowska who retired earlier this summer.
Reporting to director of registration and corporate services Dennis Roberts, Hallett takes the helm at a pivotal time for the ONS. The Treasury plans to make the ONS independent as a way of clawing back public trust in government information.
It wants to scrap the Statistics Commission, which was set up in 2000 to improve government figures, and replace it with a new governing board for the ONS.
The ONS is also about to move its headquarters from London to Newport, a move that brings internal comms challenges, acknowledged an ONS spokesman.
"The ONS is going through a very exciting period of change and I wanted to be part of it," Hallett told PRWeek.
During her time at UK Transplant, Hallett built a 13-strong comms team and helped to increase the organ transplant registration list from eight million to 13.4 million donors via a series of campaigns.
She currently reports to NHS Blood and Transplant director of comms and corporate affairs Liz Reynolds. Her reporting line changed when the organisation was merged with the National Blood Authority in October 2005.
She previously reported to UK Transplant CEO Chris Rudge - now managing and transplant director - to whom she now has a dotted reporting line.
The organisations merged following the Government's arm's-length review and now operate under the umbrella of NHS Blood and Transplant.
UK Transplant is not immediately looking to replace Hallett, pending a review of the post and its wider comms structure.