An investigation is under way after a 26 year old man was found unconscious with head injuries in central Bicester.
Officers were called by the ambulance service at about 3.55am this morning following reports of an assault in The Causeway.
Officers attended the scene, where the man had sustained serious head injuries. He was taken to hospital for treatment, where he remains in a critical condition.
Forensic officers are at the scene, which was cordoned off earlier today.
A 32 year old man has been arrested on suspicion of GBH and remains in police custody.
Investigating officer, Detective Inspector Paul Powell, of Abingdon Local CID, said the victim is in hospital with serious head injuries