Public-spirited people in Bicester have helped police make five arrests.
In the first incident a woman raised the alarm when she saw a man and a woman acting suspiciously in the town's Crown Walk car park.
Officers arrived and arrested the couple.
A man has been charged with attempting to steal.
Insp Ian Edwards, of Bicester police, said they would be questioned about other, similar incidents in the town.
He is appealing for people who have seen anything suspicious to come forward.
A motorist spotted three men changing the number plates of a BMW car on the A41 near Launton.
He contacted police on his mobile phone and the men were arrested.
They have been charged with conspiracy to steal.
Insp Edwards said the callers had showed fantastic public spirit.
He said: "Their actions are to be congratulated."
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