The incident occurred at around 10pm on Thursday at the Royal Kebab House in Broad Street.
An altercation began inside in which the man, aged 46, was assaulted by up to six youths, being punched and kicked multiple times.
The altercation spilled out on to Broad Street.
The main offender is described as white, around 18-years-old, 6ft tall and was wearing a black bomber jacket.
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The others who were involved are all described as white and in their teens, two females and three males.
The victim suffered a suspected fractured eye socket, a broken nose, bruising to the head and cuts which required hospital treatment.
He has since been discharged.
Investigating officer PC Cameron Morgan, based at Banbury police station, said: "This was a violent and seemingly unprovoked attack on a vulnerable man, who has sustained some quite significant injuries.
"I believe that Broad Street will have been fairly busy at this time of night, and I am appealing to anybody who witnessed the assault, or the aftermath in the street, to call the 24-hour non-emergency number 101, quoting reference 43190306902.
"I am also asking anybody who may have been driving in the area who may have dash-cam footage, to please check this and contact us if it has captured anything relating to this incident.
"If you don't wish to speak directly with police, you can also make a report online, or call the independent charity Crimestoppers anonymously on 0800 555 111."