Spoof adverts placed in bus shelters on Tuesday have been removed.
Protesting artists taking part in “Brandalism” swapped real advertisments with their own satirical works.
One poster read “Plan for a Healthy Retirement” and featured a picture of a doctor handing an elderly woman a gun.
It appeared at a bus shelter in Abingdon Road on Tuesday morning.
Media company Clear Channel, which manages the advertising on most bus shelters in Oxford, confirmed it had reported the matter to police.
Brandalism is described on brandalism.org.uk as “a revolt against corporate control of the visual realm”.