The mental health trust was created in 2003 by the unification of several organisations, including Broadmoor Hospital Authority, Ealing, Hammersmith & Fulham NHS Trust and, most recently, Hounslow Mental Health Trust.
'Because it is a relatively new trust, the challenge is to build a new identity and reputation,' said O'Brien.
The trust comprises more than 30 sites and 4,000 staff. Services include the high-security hospital at Broadmoor in Berkshire.
'High-security services are always controversial - that is a given, so we will always be in the public eye,' said O'Brien.
Hammersmith Hospitals Trust is renowned for its R&D, taking 11 per cent of the NHS's national budget for research.
It hit the headlines in 2003 when it treated Tony Blair for chest pains.
Reporting to chief executive Simon Crawford, O'Brien - who was at Hammersmith for ten years - manages two staff.
She joins as director of PR and comms. Press and PR manager Paul Robertson left in January.