The census, the UK's largest population count, is due to take place in 2011. The ONS plans and carries out the census for England and Wales, while the General Register Office for Scotland (GROS) handles the census for Scotland.
An ONS spokeswoman confirmed it would be looking for PR support later this year, but could not provide further details.
The GROS has already taken on Scottish shop Barkers to handle its PR activity in the run-up to the population count.
The agency has been brought on board to raise awareness of the census through a Scotland-wide campaign targeting hard to reach audiences.
The three-year contract includes planning and preparation for the 2009 rehearsal, which will include 50,000 households. Responses will help GROS assess whether the census questions and delivery are suitable.
The rehearsal for England and Wales will also take place in 2009 in Lancaster, the London Borough of Newham and the Isle of Anglesey in North Wales.
A GROS spokeswoman said: 'Communicating with every household in Scotland is a big challenge. We chose the proposals that demonstrated a detailed strategic understanding of the tasks ahead and the issues involved.'
The Northern Ireland Statistics and Research Agency handles the Northern Ireland census. A spokeswoman said the body would not hire PR support until 2009.