The offender is described as a white man around 6ft tall, with dark hair and of skinny build.
He is believed to be in his mid-thirties and spoke with an eastern European accent, possibly Polish.
He was wearing black combat trousers and a dark grey jacket.
Investigating officer Detective Constable Hugh Bailey, based at Reading police station, said: “I am appealing to anybody who believes that they witnessed this incident to please make contact with police.
“This would have happened at a time of day that would have been busy on Oxford Road, and so I am also urging anybody who was driving in the area and may have dash-cam footage, to please check this and get in touch if it has captured anything that can help this investigation.