South Central Ambulance Service said it called in the service from 21 December to ensure it met critical calls.
Fire appliances, with trained trauma technicians and defibrillators onboard, were sent if it was quickest.
In a two-and-a-half week period over Christmas the fire service attended 240 critical calls.
Firefighters responded to a medical call every 30 minutes at peak times and performed CPR on numerous occasions.
Oxfordshire Fire and Rescue's chief fire officer Dave Etheridge said: "Our staff have made this transition successfully, from the call handling to arriving at scene and assessing the patient up to the final handover to ambulance crews."
The fire service, which is continuing to answer ambulance calls, said sharing responsibilities had not affected its ability to tackle its own emergencies.
Fire engines have been used to respond to emergency ambulance call outs in Oxfordshire following a surge in demand over the Christmas period.