Sosai is the highest title in the Kyokushin karate system, meaning «founder of the organization.» In world Kyokushin, this title is associated exclusively with Masutatsu Oyama — the creator of Kyokushinkai Karate.

It was Sosai Oyama who made Kyokushin one of the most well-known karate styles in the world. He developed the full-contact system, introduced the World Open Championships, and popularized the hyakunin kumite — the 100-man fight challenge.

Oyama’s philosophy was built around the idea of overcoming oneself. He believed that a true karateka must not only be physically strong, but also possess iron discipline, patience, and respect for others.

In many Kyokushin dojos, a portrait of Sosai is displayed in the main part of the hall. Before training, students bow in his direction to show respect for the founder of the school.

After the death of Masutatsu Oyama in 1994, Kyokushin split into several organizations; however, the figure of Sosai Oyama remains central to all branches of Kyokushin.