The 20-year-old, who also has a bronze in parallel bars for good measure, scored the same 14.533 points as Colombia's Angel Barajas, but took gold on the strength of his higher execution mark.
China's Zhang Boheng and Taiwan's Tang Chia-Hung shared bronze.
Oka, basking in the glow of his golden haul, said he felt the pressure drop off once he had helped Japan to the team title.
"My favourite one is of course the gold medal in the team final, because that's what we've been working so hard for. It's a collective result," said Oka.
"After the team final, the pressure was off my shoulders. I was just able to perform with less pressure" he commented.
Teenager Barajas made it a red-letter day for Colombia with the country's first-ever Olympic gymnastics medal.
"I know I did a good job and wrote history for Colombia by doing so. To be a 17-year-old in the Olympics is a crazy experience, but it is more important to just enjoy it out there" he said.
The final was marked by several falls and fumbled dismounts, with Tang and in particular Japan's Takaaki Sugino hitting the deck to gasps from the crowd.
Belying his inexperience, Oka kept his nerve and crucially his grip on the 2.8 metre-high bar to top the standings.
His score was matched by Barajas, but he was awarded 7.933 for execution compared to Oka's 8.633.
In contrast, Zhang and Tang were inseparable on 13.966 and an identical 7.466 execution mark left them tied for third.
Defending champion from the Tokyo Games, Daiki Hashimoto, failed to qualify for the final.
In a sign of how errors played their part in the 2024 final, Oka's score in Paris would only have won bronze at the Covid-delayed Games three years ago.
