About 1,800 homes are still without power after a fault with overhead power cables.
The fault with the high voltage network feeding into the Yarnton substation left homes and businesses in Bicester, Kidlington, Weston on the Green and Islip without electricity from 4.30pm.
Traffic lights were also affected, leading to congestion on some routes - including the junction of the A34 and M40 at Weston on the Green.
Power was restored to the traffic lights at the busy junction at 8pm, and is expected to be restored to most homes during the night.
Southern Electric spokesman Penny Porter said: "Because of the dark it took a while to find the cause of the problem. We would like to apologise for the inconvenience."
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