Colombian teenager Angel Barajas took silver and Taiwan's Tang Chia-hung and China's Zhang Boheng shared bronze despite botched performances.
Oka, brimming with confidence after he won gold in the team and all-around events as well as a bronze on the parallel bars earlier in the day, stuck his landing for a score of 14.533.
While 17-year-old Barajas equalled his mark, Oka remained ahead of him due to his higher execution score.
More than half of the field came off the apparatus mid routine or suffered falls and stumbles on dismount in what was a messy conclusion to the men's artistic gymnastics in Paris.
A new champion was guaranteed in the event after Tokyo gold medallist Daiki Hashimoto of Japan failed to qualify for the final.
More to follow.
