The offender was white, over 6ft tall and of stocky build. He had short dark hair and was wearing a white Nike t-shirt with a small blue tick on it and black jogging bottoms or jeans.
Investigating officer Det Sgt Mark Stalder from Oxford Force CID said: “At the moment we would like to talk to anyone who saw this happening and we urge the man involved to come forward to police.
“If anyone saw anyone acting suspiciously yesterday morning, please do get in touch.
“Incidents like this are incredibly rare and I want to reassure the community we are doing everything to catch this offender.”
Anyone with information can contact Det Sgt Stalder on 101.