Oka met Takaaki Kidani, president of Bushiroad in 2013, and was signed to New Japan Pro-Wrestling (NJPW) shortly after, but didn't begin training until 2015 as he was still in university at the time.

[3] 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.

[11] On April 13 at Lion's Gate Project 4, he teamed with his trainer Yuji Nagata to defeat Katsuya Kitamura and Manabu Nakanishi.

[15] 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).

On October 16, 2020, O-Kharn, now going by the slightly modified ring name Great-O-Khan,[17] returned to NJPW during G1 Climax 30, helping Will Ospreay defeat Kazuchika Okada.

[18] In November 2020, O-Khan and new member of The Empire, Jeff Cobb would enter World Tag League where they finished with a record of 5 wins and 4 losses.

Following this O-Khan feuded with Tetsuya Naito culminating in a singles match at the 26th April Road To Wrestling Dontaku event.

At Power Struggle, O-Khan unsuccessfully challenged Toru Yano in an amateur rules match for the 2021 KOPW provisional championship.

[23] In September, O-Khan made his debut for Pro Wrestling Noah, teaming with The Great Muta and Nosawa Rongai to defeat Kongo (Tadasuke, Manabu Soya and Kenoh), at the finals of the N-1 Victory.

[25] The following month, O-Khan once again teamed with Aaron Henare in the World Tag League, finishing with 10 points, failing to advance to the tournament finals.

[26][27] On December 17, O-Khan returned to RevPro winning the Undisputed British Heavyweight Championship, defeating Zak Zodiac, who was replacing champion Ricky Knight Jr.[28] On January 4, 2023 at Wrestle Kingdom 17, O-Khan entered the New Japan Rambo, lasting till the final 4 and advancing to the four-way match the following day, for the Provisional KOPW Championship.

[30] Later that month, O-Khan faced Takagi for the provisional title, at The New Beginning in Nagoya, losing in a Mixed Martial Arts rules match.

[34] In July, O-Khan lost the Undisputed British Heavyweight Championship to Michael Oku, ending his reign at 204 days.

[37] O-Khan attacked Takagi, during a post-match interview, before forcing him to lick his boots, as he promised an "execution" at the hands of the United Empire.

The first fall would be held under a ten minute time limit most cover rules, with every pinfall attempt counting as a point in the scenario.

Two days later, it was announced that O-Khan and Loa would face each other in a Rural Revitalization Match in Hamamatsu, following the result of a fan poll.

Four days later, it was announced that O-Khan and Uemura would face each other in a Rural Revitalization Match in Hamamatsu, following the result of a fan poll.

[50][1] On April 4, 2022, Oka was given a letter of appreciation by Kawasaki City police after saving a teenage girl who had been grabbed by a drunk man at JR Musashi Kosugi Station on March 29, 2022.