Blue light versions of modern cars and 4x4s are being introduced to enable paramedics to get to emergencies more quickly, often arriving before an ambulance or police.
As part of the changes, the trust has taken delivery of 15 Nissan Primera five-door diesels and three X-trail diesels to work as rapid response paramedic vehicles.
The new additions to its fleet are fully equipped with items such as a resuscitation kit and oxygen.
Trust divisional manager John Willis said: 'The emergency response vehicles have a crucial part to play in the Oxford Ambulance NHS Trust service and we test-drove many makes and models before choosing Nissan.'
Nissan fleet sales director Dave Murfitt said: 'The provision of specialist vehicles such as these continues to be a successful part of our fleet offering. Working with specialist installers of equipment necessary for emergency vehicles, Nissan is carving a niche for itself in this demanding market.'