The woman, in her 30s, was approached from behind and struck in the face before being sexually assaulted in Langdale Common, Witney at about 03:00 BST on Saturday.
Police said the offender, who wore a black jacket, fled after possibly being disturbed by other people.
Officers have appealed for information from anyone in the area at the time.
Det Insp Larry Johnson, from Thames Valley Police, said: "I would also like to speak to anyone who was in the area of Witan Way, close to the entrance to Langdale Common and near a kebab van, as they may also have information which could assist our investigation."
"I understand this incident could cause the community some concerns, therefore additional patrols are taking place," he added.
A man is being sought by police after a woman was sexually assaulted on parkland in Oxfordshire.