Almost half of licensed premises in Thame tested by police were caught selling alcohol to underage drinkers.
Police carried out a test-purchasing operation on 11 pubs, bars and shops in Thame on Friday May 9.
Details were only released yesterday.
The five failures were pubs Cross Keys, Nags Head and Six Bells, bar Ciao Baby and Sainsbury's supermarket.
Police issued £80 fines and summoned the management to attend a meeting with South Oxfordshire District Council to tackle underage sales.
Licensing officer Tony Cope said: "We will work with those premises that have failed for the first time to make sure their staff are trained correctly and the right processes are in place.
"If a licensed premise fails these operations three times, then we consider reviewing their licence and it can be revoked."
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