At around 2am Sunday, September 22, the victim, a 32-year-old man, was walking home along an alleyway between Edmunds Road and Bretch Hill when an unknown man approached him from behind and threatened him with a knife.
The offender stole an amount of cash from the victim's wallet before discarding it on the floor and running off.
Investigating officer PC Jacob Wilson based at Bicester police station said: "This was a frightening experience for the victim.
"I am appealing for witnesses to this incident to come forward and help us with our enquiries while we investigate.
"I am particularly keen to speak to a cyclist who cycled passed the victim on Edmunds Road shortly before the incident. I believe this person may have vital information about the incident and could help with this investigation.
"Anyone with information is asked to call 101, quoting 43190292997, make a report online, or call the independent charity Crimestoppers anonymously on 0800 555 111."