In the United Kingdom a Chief Ambulance Officer (CAO) is the highest ranking, medically qualified officer within a public NHS Trust Ambulance Service or private Ambulance Service.
Their role is comparable to that of a Chief Constable within the UK Police Force or Chief Fire Officer within the UK Fire & Rescue Service
CAOs are the operational and administrative heads of the Ambulance Service and take command of all ambulance personnel at large operational incidents. They also set policy and targets (along with a board of directors) and are responsible for the day to day running of the Service.
The title of COA is normally only used by medically qualified Ambulance Paramedics who have risen through the ranks to become Chief of the Ambulance Service and who have served operationally on the front line.
In recent times Several NHS Ambulance Trusts have employed non-medically qualified civilians as Chief Ambulance Officers. They are referred to as Chief Executive to denote that they are not medically trained, or have come from a non-Ambulance Service background.
Confusingly the rank of CAO is now largely being phased out and both medically qualified Chief Officers and civilian Chief Officers are styled as "Chief Executive". In this case medically qualified CEOs ordinarily wear a uniform whereas civilian CEOs do not.