Police are investigating.
A 15-year-old boy was robbed on Good Friday at around 3.30pm.
He walking down the alleyway in Boundary Brook Road near Larkrise Primary School when a man with a knife grabbed him by the arm and then stole some money from him.
The man is described as black, around 6 foot tall and aged in his twenties. He was wearing a navy tracksuit with the hood up and was also wearing a black face mask.
Detective Sergeant Richard Allmond based in Oxford, said: “This was a distressing incident for the boy involved.
“It was broad daylight when this took place, and although the boy was in an alley, we believe others will have seen what happened.
“If you have any information about this incident or you can help us with our enquiries, please contact police by either calling 101 or making a report online using the reference number 43210140288."
The victim wasn't hurt in the robbery.