She was approached by a teenage boy she didn't know in Hanborough Park.
Police are appealing for witnesses to the attack near the play area, behind the pavilion, on Monday.
It happened shortly after 5pm, the victim managed to get away from the offender and ran across the playing field.
PC Laura Oakes based at Witney police station, said: “I would especially like to trace the man who was seen running with a dog at the time of the offence. The dog was a grey lurcher or greyhound.
“The man was wearing a running vest and jogging bottoms and could have vital information for our investigation. He is not being treated as a suspect.
“I believe it was the presence of this man that actually led the assault being stopped, and so I am appealing for him to make contact with police.
“If you do have information, please contact the 24-hour police call centre on 101 or you can make a report online here quoting the reference number 43180277129."
The boy police are looking for is described as white, between 5ft 9ins and 6ft tall and aged between 15 and 17-years-old.
He has dark brown hair and a spotty face with red patches. He was wearing a navy blue polo shirt and black Adidas trainers with white soles.