Election officials set up a temporary polling booth in a car boot after the keyholder to the normal station overslept.
Polls were due to open at 7am on Thursday at the Oxford Centre for Mission Studies, however voters arrived to find the doors locked.
Oxford City Council confirmed the keyholder had overslept, and the presiding officer used their car as a makeshift polling station as is “standard procedure”.
A spokesman said everyone waiting had been able to vote, and the building was open within 15 minutes.
Voter Toby Porter said he was one of around a dozen people who cast their ballots in the pop-up station.
“We found it funny. Everyone was enjoying the novelty,” he told the PA news agency.
A council spokesman said: “The keyholder overslept and for a short time at 7am electors were voting using the PO’s (presiding officer’s) car.
“This is standard procedure when a station building isn’t open on time and part of the training we give them.
“The key is that ballot papers are ready to be issued at 7am, wherever that may be. The building was open within 15 minutes.”
Laura Lock, deputy chief executive of the Association of Electoral Administrators, said some polling stations in cars are seen at each election.
“All polling station staff are trained on how to set up temporary polling stations for cases just like this,” she said.
“Unfortunately we do find key holders oversleeping, so every election we see a handful of polling stations in cars until access to the building can be sorted.”