Police are investigating.
A 35-year-old man has died after an ambulance and a pedestrian collided on the northbound carriageway, between the junctions for Peartree and Islip.
It happened at around 3:20am today.
His family have been told and are being supported by specialist officers.
South Central Ambulance Service NHS Foundation Trust says the ambulance wasn't responding to an emergency at the time and the driver has been suspended.
Mark Ainsworth, Director of Operations at SCAS, said: "On behalf of South Central Ambulance Service NHS Foundation Trust, I would like to offer my sincere condolences to the family and friends of the pedestrian who sadly died following a collision with one of our ambulances on the A34 early this morning. The ambulance was not travelling under emergency driving conditions at the time. The Trust and the ambulance crew are assisting Thames Valley Police with its investigation into the collision.
"It's normal practice in such circumstances for the driver of the vehicle to be suspended from driving duties whilst any external and internal investigations are undertaken."
Drivers and bus users are being warned of delays.
One lane of the A34 is still closed northbound between Hinksey and Peartree while the road surface repairs are completed.
We are experiencing delays of 2 hours on city services due to the A34 closure. Thank you for your patience during this time.— Oxford Bus Company (@OxfordBusCo) November 29, 2019
Thames Valley Police have appealed for witnesses and dash-cam footage following the collision. They are interested in any footage from cars travelling on the northbound carriageway in the early hours that may have seen the man.
They have also said that they believe the man walked from Oxford City Centre towards the Peartree interchange before entering the northbound carriageway.
Investigating officer, Detective Constable Rob Turpin, of the Serious Collision Investigation Unit in Bicester police station, said: “This was a tragic incident where a 35-year-old man has died after being in a collision with an ambulance.
“I would appeal for anyone who may have seen this pedestrian, a 35-year-old man, in the northbound carriageway of the A34 at a time prior to the collision taking place."