The 45-year-old was attacked by a man in a semi wooded area at the back of Cairo Mill,Greenacres around 1.45 on Thursday lunchtime.
The victim required hospital treatment for a face injury but has since been discharged.
An investigation is underway.No arrests have been made and enquiries are ongoing.
The offender is described as Asian, in his late teens, approximately 6ft 2ins tall and of a slim build with a goatee. At the time of the incident he was wearing a black tracksuit.
Detective Inspector Thomas Howard, of GMP’s Oldham district, said: “This was an unprovoked attack which left an innocent man with a nasty face injury and we are providing him with support whilst our enquiries continue.
“Whilst we believe this was an isolated incident, we have increased patrols across the area to provide further support and engagement for the community.
“The man responsible fled the scene on foot and may have run on to Beckett Street, Constantine Street, Turner Street or Wellyhole Street and I would like to appeal to anyone who witnessed the incident or has information, regardless of the significance, to get in touch with us as soon as possible.”