After formally establishing the Kyokushinkaikan in 1964, Oyama directed the organization through a period of expansion. Oyama hand-picked instructors who displayed ability in marketing the style and gaining new members. Oyama would choose an instructor to open a new dojo. The instructor would move to that town and demonstrate his karate skills in public places. After that, word of mouth would spread through the local area until the dojo had a dedicated core of students. Oyama also sent instructors to other countries such as the Netherlands (Kenji Kurosaki), Australia (Shigeo Kato and Mamoru Kaneko), the United States (Tadashi Nakamura, Shigeru Oyama and Yasuhiko Oyama, Miyuki Miura), Great Britain (Steve Arneil), Canada (Tatsuji Nakamura) and Brazil (Seiji Isobe) to spread Kyokushin in the same way. Many students, including Jon Bluming, Steve Arneil, and Howard Collins, traveled to Japan to train with Oyama directly. In 1969, Oyama staged The First All-Japan Full Contact Karate Open Championships and Terutomo Yamazaki became the first champion. All-Japan Championships have been held at every year. In 1975, The First World Full Contact Karate Open Championships were held in Tokyo. World Championships have been held at four-yearly intervals since.
After Mas Oyama's death, the International Karate Organization (IKO) split into two groups, primarily due to personal conflicts over who should succeed Oyama as chairman. One group led by Shokei Matsui became known as IKO-1, and a second group led by Yukio Nishida and Sanpei became was known as IKO-2. The will was proven to be invalid in the family Court of Tokyo in 1995. Before his death, Oyama named no one as his successor, although he did mention Matsui to be the most eligible one .
In 1995 any new Kyokushin organization that claimed the name IKO, Kyokushinkaikan, were referred to by Kyokushin practitioners by numbers, such as IKO-1 (Matsui group), IKO-2 etc.
Due to this break up, many attempted to establish their own leadership. For example, IKO-2 was not organized by Oyama's family, although Chiyako Oyama was asked to succeed after her husband as Kaicho. Chiyako Oyama stepped away from the political fight and founded the Mas Oyama Memorial Foundation with her daughters, still retaining the rights to the companies that managed IKO Kyokushinkaikan during Mas Oyama's leadership.
Japan-based Kyokushinkaikan organisations
, Japanese-based organizations that claim the name "International Karate Organization" include:
- IKO Kyokushinkaikan "Sosai", organized by Mas Oyama's daughter, Kuristina Oyama, which by court order has the rights to Mas Oyama's Honbu.
- IKO Kyokushinkaikan "Matsushima", headed by Yoshikazu Matsushima.
- IKO Kyokushinkaikan "Matsui-Ha" or "Ichi-Geki", headed by Shokei (Akiyoshi) Matsui.
- IKO Kyokushinkaikan "All Japan Kyokushin Union" or "Kyokushin Rengōkai", headed by Yasuhiro Shichinohe.
- IKO Kyokushinkaikan "Sakamoto-Ha", headed by Shigenori Sakamoto.
- IKO Kyokushinkaikan "World So-Kyokushin", headed by Daigo Ohishi.
- IKO Kyokushinkaikan "Nakamura", headed by Makoto Nakamura.
Other Japan-based organisations
Other Japanese Kyokushin groups no longer officially claiming the original name of "IKO" and "Kyokushinkaikan":
- WKO (World Karate Organization) Shinkyokushinkai, headed by Kenji Midori as president.
- Kyokushin Shogakukai Foundation, Kyokushin-kan International Karate-do Organization (KIKO), headed by Hatsuo Royama as president.
- Kyokushin Karate Union Kyokushin Kenbukai. Founded by Tsuyoshi Hiroshige an apprentice of Mas Oyama.
Non Japan-based organisations
Kyokushin groups outside Japan:
- International Federation of Karate, Kyokushin (IFK) – founded by former IKO member Steve Arneil.
- Kyokushin World Federation (KWF), founded by former IKO members: Loek Hollander, Antonio Pinero and Andre Drewniak.
- Kyokushin Budokai, IBK, founded by former IKO Jon Bluming.
- International Kyokushin Union (IKU) – founded by former IKO member David Farzinzad.
- International Kyokushinkai Association (IKA) – founded by former IKO member Carllos Costa, based in Brazil.
- International Federation of Kyokushinkaikan Karate (IFKK) – founded by former IKO member Sahinbas Goksel and Malik Dilnawaz, based in Netherlands and Norway.
- International Seishin Kyokushin Karate Organization (ISKKO) – founded by Mas Oyama Shibucho for Catalunya Pere Lluis Beltran, based in Spain.
- International Kyokushinkai Karate Federation (IKKF) – founded by former IKO member Teyub Azizov, based in Azerbaijan.
- World Kyokushin Karate Federation (WKKF) – founded by IKO member Bodh Narayan Yadav, based in India.
- World Kyokushin Budokai (WKB) – founded by former IKO member Pedro Roiz, based in Spain
Oyama's widow died in June 2006 after a long illness.
Mas Oyama's youngest daughter, Kikuko (also known as Kuristina) now oversees the management of the original IKO Kyokushin kaikan Honbu. She also published a book in 2010, a collective memoir of Mas Oyama and his teachings.
In May 2012, the Japanese Patent Office granted the Kyokushin related trademarks to Kikuko Kuristina Oyama, after years of long court battle.
She has internationally trademarked and copyrighted her father's work and devotes the proceeds to various charities.