A 42-year-old man remains in hospital with head injuries after he was cut from his car after hitting a tree.
Police, fire and ambulance services rushed to the scene on East Claydon Road, Winslow, just before 7am today.
The man, who was travelling towards East Claydon, had to be cut from his Red Hyundi i10 before being flown to the John Radcliffe Hospital in Oxford.
Police would like to speak to anyone with information.
Investigating officer, PC Christopher Ruvolo-Jones of the Roads Policing Unit, said:“I would like to appeal to anyone who either witnessed the collision, or saw the vehicle before the incident to contact me via the Thames Valley Police Enquiry Centre on 101.”
Alternatively, contact Crimestoppers anonymously on 0800 555 111 or online at www.crimestoppers-uk.org