A 20-year-old woman was sexually assaulted while attending a student sports night at the O2 Academy in Oxford.
At about 1.30am, the victim was sexually assaulted while she was on the ground-floor dance area near the stage.
Police said the attacker, a man, is white, about 6ft, in his late teens to mid twenties, of medium build and has mousey or brown coloured hair with a quiff and short sides.
He was wearing a blue long-sleeved top and dark trousers.
The incident occurred on November 3, but police have only just released details of the crime.
Officers have now released CCTV pictures of a potential witness.
A student night called Fishies Sports Night was at O2 on the night of the offence.
Investigating officer, Det Con James Macaro of Force CID, said: "We are appealing directly to the people in the CCTV images to please contact Thames Valley Police as we believe they are potential witnesses to the incident.
"Alternatively, if you know the people in the CCTV or have any other information about the incident please call our 24-hour enquiry centre on 101."
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