Tomoyuki Oka, better known by his ring name Great-O-Khan, is a Japanese professional wrestler signed to New Japan Pro-Wrestling (NJPW), where he is a member of the United Empire stable. A former NJPW World Television Champion and final official King of Pro Wrestling Champion, he is also a four-time IWGP Tag Team Champion with Jeff Cobb (2x), Aaron Henare, and Callum Newman. In 2026, he won the NEVER Openweight 6-Man Tag Team Championship alongside Henare and Will Ospreay. In recognition of his in-ring ability, he received Tokyo Sports’ Technique Award in 2021, Fighting Spirit Award in 2022, and Best Tag Team Award in 2022 alongside United Empre stablemate Jeff Cobb.

Oka began training with NJPW in 2015 and debuted in 2016 as part of the company’s Young Lion system before heading to the United Kingdom for an excursion with Revolution Pro Wrestling (RevPro), where he reinvented himself as Great-O-Khan and later captured the Undisputed British Heavyweight Championship. He returned to NJPW in 2020 as the first major member of Will Ospreay’s United Empire faction and quickly became one of the promotion’s most distinctive heavyweights, blending amateur wrestling, submission grappling, and his eccentric imperial persona. Known for his unconventional matches and crowd interaction, O-Khan gained further momentum in 2024 after reaching the G1 Climax playoffs and unsuccessfully challenging Tetsuya Naito for the IWGP World Heavyweight Championship. In 2025, he was ranked No. 137 in the Pro Wrestling Illustrated PWI 500.