She said: 'Around 800 staff have left, the HQ has moved from London to Luton, Lunn Poly has rebranded to Thomson, and Britannia has become Thomsonfly.
'Staff need to know the business is moving forward, and it is the job of internal comms to boost morale and inspire confidence.'
O'Reilly said she would be reviewing the 'tone and channels' used by TUI in its staff comms, and would recommend a single newsletter.
Her 14-strong team will also be responsible for the bulk of TUI's consumer PR when four-year incumbent Lexis PR reaches the end of its contract next month.
BGB Communications will continue to handle comms for Thomsonfly.
The head of comms role was created when head of internal comms Lisa Tungate decided not to return from maternity leave last month.
O'Reilly reports to sales and marketing director Miles Morgan.