Posters have been put up at two hospitals reminding staff to make sure they are treating the correct patient after a series of mix-ups.
Oxford University Hospitals NHS trust introduced the signs in staff areas at the John Radcliffe and Churchill following four "never events".
They included the wrong person receiving an endoscopy, where a camera is inserted into a patient's throat.
The trust has apologised for the mistakes.
In board papers it said making sure staff were treating "the correct patient every time" has been made a priority.
It said: "We chose this priority following a number of incidents, particularly in radiology, where the wrong patient received a test or procedure in the previous year."
"Never events" are classified as "serious incidents that are entirely preventable", and there were eight at the trust in 2017-18.
They included a "sizing trial cup" being left inside a patient after surgery, requiring a second operation to remove it, and a patient getting the wrong injections.
Medical director Dr Tony Berendt said: "These posters were put up in all staff areas recently as part of a suite of actions to remind staff to thoroughly check a patient's identity.
"The posters are part of a programme of work to ensure that patients receive the safest care possible."
A spokesman added that while the current posters are internal, it is working on versions for public areas.