A man stole a “substantial amount” of money from The Co-operative Food store in Eynsham on Wednesday evening.
At 7.30pm, a man entered the store on High Street brandishing an unidentified object and threatened staff with it.
Charlotte Redman, of Thames Valley Police, said it was impossible to identify the object because it was covered, but said the robber was able to get away with the cash.
She urged witnesses to call police on 101.
The Co-operative declined to comment.
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