The incident on Monday morning in Witney, has also left two other 14 year-old girls and a man in his 40s in hospital.
An 18 year-old man has been arrested by police on suspicion of causing death by dangerous driving.
It's understood one of the girls has serious head injuries whilst the other has escaped major injury.
The male pedestrian hurt his hip after it's thought a car mounted the pavement on Curbridge Road, just yards from Our Lady and St. Hugh Catholic Church.
Parish priest Father Patrick Broun tells Premier local people are devastated and appealed for prayers.
Inspector Paul Winks from Thames Valley Police said: "At the moment we're trying to establish what's happened here.
"We've had forensic specialists at the scene, this is just the early stages of the investigation.
"We need to speak to witnesses, look for CCTV evidence and evidence on the vehicle itself that may indicate exactly how this happened.
"I'm absolutely determined to get to the bottom of what's happened here."
All three of the girls went to the nearby Henry Box Schoool. The headteacher there confirmed the death 'with deep regret'.
A statement from Wendy Hemmingsley said: "The school community is deeply saddened by this news and our thoughts are with her family and friends at this difficult time. Support is being offered to students and staff."
The girl who died was a former pupil at Our Lady of Lourdes Catholic Primary School.
Teachers there are offering support to students including the girl's younger brother who is in year 5.
Inspector Winks added: "The family is being supported. This is an awful tragedy and our thoughts are with that family and the other families involved in this."
Curbridge Road was closed both ways between Church View Road and A415 Ducklington Lane to allow investigation work to take place.
Our Lady and St. Hugh Catholic Church will remain open for 'as long as is needed' to support the community.