Mas Oyama, born Choi Yeong-eui in 1923 in Korea, was the founder of Kyokushin Karate, the first and most influential style of full-contact karate emphasizing physical conditioning, breaking techniques, and practical combat.[1][2][4] At age 15, he moved to Japan in 1938 aspiring to become an aviator, but immersed himself in martial arts, earning a nidan (second-degree black belt) by 18 and training rigorously under influences like Gichin Funakoshi and Nei-chu So.[3][6][8] Post-WWII, Oyama won the karate division of Japan's first National Martial Arts Championships in 1947.[1][3] Seeking deeper mastery, he underwent 18 months of solitary training on Mount Kiyosumi in 1948, enduring extreme regimens like waterfall stands, stone-breaking, and tree-punching.[1][3] Famous for feats like killing 47 bulls with bare hands—four instantly—he demonstrated karate's destructive power.[1] In 1953, he opened his first dojo in Tokyo, initially teaching Goju-ryu before developing Kyokushin ("Ultimate Truth").[3][5] By 1964, he established the International Karate Organization Kyokushin Kaikan (IKO) headquarters, expanding to over 72 branches in 16 countries by 1960 and eventually 120+ nations with 12 million members.[2][3][4] Oyama pioneered full-contact tournaments: the first All-Japan Open in 1969 (won by Terutomo Yamazaki) and World Open in 1975, held quadrennially and open to all styles.[2][4][5] He authored the bestseller "What Is Karate?" (1958, 500,000 copies), taught U.S. military and FBI personnel, judged events like the 1959 Hawaiian Tournament, and received Brazil's Cultural Award in 1985.[1] Oyama died of lung cancer on April 26, 1994, at age 70 in Tokyo; his widow founded a legacy organization.[4] Kyokushin remains a dominant force in martial arts, influencing styles like Kudo.[5]