A motorcyclist had to be flown to hospital after colliding with a car near Long Crendon on Sunday.
The 22-year-old male, of Chinnor, was taken to John Radcliffe Hospital after the crash on the B4011 at 4.29pm.
The motorcyclist is believed to have sustained serious injuries but has now been discharged from hospital while the car driver, a 47 year old man from Thame, suffered minor injuries.
The collision involved a white Honda sports bike and a silver VW Passatt.
Investigating officer PC John Belson said: “We believe there was another motorcyclist travelling along the road at the time of the collision who did not stop at the scene. We would like to speak to them as they may be an important witness.
“They were riding a BMW Adventure-style endure bike, which was red, white and blue with a Red Bull sticker on the machine.
“I would also like to speak to anyone else who witnessed the collision or has information which could help the investigation.”
If you can help, please call PC Belson via the 24-hour Police Enquiry Centre on 101.