After formally establishing the International Kyokushin Karate Organization – Kyokushinkaikan in 1964, Sosai Oyama would direct the organization through a period of expansion. Sosai Oyama hand-picked instructors who possessed ability in marketing the Kyokushin Karate style and gaining new members. Sosai Oyama would choose an instructor to open schools in another town or city in Japan. The instructor would move to that town and usually demonstrate his karate skills in public places, such as at the civic gymnasium, the local police gym (where many judo students would practice), a local park, or conduct martial arts demonstrations at local festivals or school events. In this way, the instructor would soon gain students for their new Dojo.
After that, word of mouth would spread through the local area until the Dojo had a dedicated core of students. Sosai Oyama also sent instructors to other countries such as the Netherlands (Kenji Kurosaki), Australia (Shigeo Kato), the United States of America (Tadashi Nakamura, Shigeru Oyama and Yasuhiko Oyama, Miyuki Miura) and Brazil (Seiji Isobe) to spread Kyokushin in the same way. In addition, numerous students began to travel to Japan to train with Sosai Oyama, consequently returning to their country to spread the art.
In 1969, Sosai Oyama staged The First All-Japan Full Contact Karate Open Championships which took Japan by storm and Terutomo Yamazaki became the first champion. All-Japan Championships have been held at every year since. Also in 1975, The First World Full Contact Karate Open Championships were held in Tokyo. World Championships have been held at four-yearly intervals since.
Upon Sosai Oyama’s death in 1994, several groups broke away from the International Karate Organization (IKO) Honbu, primarily due to personal conflicts over who should succeed Sosai Oyama as chairman. As a supposed will was proven to be invalid in the family Court of Tokyo in 1994, any claim to that will indicating the true intention of Sosai Oyama was nullified. Before his death, Sosai Oyama named no one as his successor although many now claim to be the rightful leader of his organization.
Existing as a single organization under the leadership of the founder, Mas Oyama, the Kyokushin organization divided into several groups after the Master’s passing, each claiming their own authority as representing the original Honbu. The groups are often referred to as “IKO1″, IKO2″, IKO3″, etc., although those are not their official names. The different organizations often shun each other and generally refuse to recognize each other as legitimate organizations representing the original Kyokushin organization.
Sosai Oyama’s widow died in June 2006 after a long illness. According to the Japanese legal system, the Custodian of Mas Oyama’s intellectual property and legacy is the youngest of his daughters, Kikuko (also known as Kuristina) through inheritance, who now oversees the management of the original IKO Kyokushinkaikan Honbu, although not directly involved in karate teaching. She also published a book in 2010.
In May 2012, the Japanese Patent Office granted the Kyokushinkai related trademarks to Kikuko Oyama, after years of long court battle.