Police have described his injuries as life-changing - and have arrested two men in connection with the assault.
As we reported earlier, police cordoned off a property in Middleton Road, Banbury, while they investigated the incident.
In the last few minutes, police have released some information about the attack.
A Thames Valley Police spokesperson said: “We received a report of an assault on Middleton Road, Banbury, from South Central Ambulance Service at around 1.35am this morning (April 29).
“The victim, a man aged in his fifties, had been found in the street by a member of the public with life-changing head injuries.
“He was taken to the John Radcliffe Hospital for further treatment, where he remains.
“A 42-year-old man from Banbury has been arrested on suspicion of wounding with intent and a 39-year-old man of no fixed abode has been arrested on suspicion of causing grievous bodily harm with intent – they remain in police custody.
“Anyone with information to call 101 or make a report on our website, quoting reference number 43220183330.
“Alternatively, contact independent charity Crimestoppers anonymously by calling 0800 555 111 or via its website
“A scene watch in place and we would like to thank the public for their patience if this causes any inconvenience.
“Members of the public are likely to see an increased police presence while we continue to investigate and we would advise anyone with any concerns to please speak to one of our uniformed officers.”