Thames Valley Police are appealing for witnesses following a fatal collision in Upper Arncott on Saturday.
At about 4.55pm, a two-year-old girl was in collision with a reversing car on the Oaksview Caravan Park.
The girl was taken to the Bicester Community Hospital and the John Radcliffe Hospital in Oxford, but was declared dead shortly after her arrival.
Investigator Stuart Whitworth said: “This is a tragic incident in which a two-year-old girl has lost her life.
“I ask anyone with any information on the collision to give us a call immediately.”
A 49-year-0ld man from Upper Arncott was arrested on suspicion of causing death by careless driving and was released on bail until March 31.
Anyone with information can contact PC Thorne via the 24-hour Thames Valley Police Enquiry Centre on 101.