FleishmanHillard has already relocated to the new premises, called Bankside 2 and Bankside 3, on Southwark Street. Fishburn is set to move in the autumn.
Omnicom told PRWeek that many of its UK-based advertising, branding, production and CRM agencies have moved or are in the process of moving into the development.
Portland recently moved into new offices on the Strand and will not be moving to Bankside.
Omnicom leased Bankside 2 and Bankside 3 from Royal Bank of Scotland at the end of 2014. The two properties total 370,000 sq ft of space.
The lease will run until 2027. Average rent is believed to be in excess of £45/sq ft.
Omnicom Group reported flat organic revenue growth of 0.3 per cent in the second quarter for its PR agencies, compared with the previous year.