It happened at the Premier Store and post office Barnards Way at around 3.30pm on Tuesday.
The man approached the counter and pointed a firearm at the staff member behind the till and demanded that he hand over money.
After taking the cash from the till he left, walking towards Charlton Village Road.
No one was hurt during the incident and police have released a CCTV image of a man officers would like to speak to.
Investigating officer Detective Constable Paul Geddert, based at Abingdon police station, said: “I am releasing this image as I believe someone will recognise his distinctive clothing or may have spotted him as his clothing would have been out of place on such a hot and sunny day.
“This is a concerning incident that took place during the daytime and I would like to reassure the community that we are conducting a thorough investigation into this offence and are following up on a number of leads.”