An 18-year-old had his jaw broken after he tried to help a woman involved in a row in Hythe Bridge Street.
The attack happened between midnight and 3am on Sunday.
Last night the man was still being treated for his injuries at the John Radcliffe Hospital.
Det Ch Insp Paul Hayles said: “This was a busy area at the time the incident occurred and I know there would have been a lot of potential witnesses.
“If you witnessed this assault then you need to come forward and tell us what you saw.”
Call the Oxford CID team on 101 or Crimestoppers on 0800 555111.
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