Tomoyuki Oka

Oka began training with New Japan Pro Wrestling (NJPW) in 2015, and made his in-ring debut on February 25, 2016, wrestling fellow rookie Katsuya Kitamura to a draw in a dark match at Lion's Gate Project 1.[5] Oka and Kitamura wrestled to another draw in a dark match on September 1 at Lion's Gate Project 3.[6] Oka made his televised debut on January 3, 2017, losing to his trainer Yuji Nagata.[7] Oka once again lost to Nagata on January 27,[8] January 31,[9] and February 2.[10] On February 9, Oka, Yoshitatsu and Henare were defeated by Nagata, Hirai Kawato and Jyushin Thunder Liger.[11] Oka's first victory came on February 21, when he defeated Henare via referee stoppage after Henare suffered an ankle injury during their match.[12] On April 13 at Lion's Gate Project 4, he teamed with his trainer Yuji Nagata to defeat Katsuya Kitamura and Manabu Nakanishi.[13] On July 22, he and Nagata defeated Kitamura and Hiroshi Tanahashi.[14] Oka then formed a tag team named "Monster Rage" with Kitamura.[15] From October 12 to December 21, Oka took part in the 2017 Young Lion Cup, where he finished third with a record of three wins and two losses.[16] Oka and Kitamura made their debuts in All Japan Pro Wrestling (AJPW) on July 5, 2017, losing to Wild Burning (Jun Akiyama and Takao Omori).[17]

On June 30, 2018, Oka debuted a new look and ring name, Great-O-Kharn. He defeated Shota Umino with a new finisher, a Mongolian chop delivered from the second turnbuckle. Great O-Kharn started his excursion in the United Kingdom with New Japan affiliate Revolution Pro Wrestling and has started with 3 impressive wins and has since reinvented his finisher into a Crossbow submission. At Revolution Pro Wrestling's Summer Sizzler O-Kharn defeated American Superstar Shane Strickland and continued his winning streak into the Rev Pro tapings. On October 13, 2019, The Great O-Kharn teamed with Rampage Brown to win the RPW Tag Titles. A month later, on November 24, O-Kharn and Brown unified the RPW Tag Titles with the SWE Tag Titles when they defeated The Moonlight Express.

On October 16, 2020, O-Kharn, now going by the slightly modified ring name Great-O-Khan,[18] returned to NJPW during G1 Climax 30, helping Will Ospreay defeat Kazuchika Okada. He joined Ospreay and his girlfriend, Bea Priestley, in a new faction, The Empire.[19] The faction would rename itself United Empire in January 2021.