Oyama was born as Choi Young-Eui (최영의) in Gimje, South Korea, during Japanese occupation. At a young age he was sent to Manchuria, Northeast China to live on his sister's farm. Oyama began studying Chinese martial arts at age 9 from a Chinese farmer who was working on the farm. His family name was Lee and Oyama said he was his very first teacher. The story of the young Oyama's life is written in his earlier books.
In March 1938, Oyama left for Japan following his brother who enrolled in the Yamanashi Aviation School Imperial Japanese Army aviation school. Sometime during his time in Japan, Choi Young-Eui chose his Japanese name, Oyama Masutatsu (大山 倍達), which is a transliteration of Baedal (倍達). Baedal was an ancient Korean kingdom known in Japan during Oyama's time as "Ancient Joseon".
One story of Oyama's youth involves Lee giving young Oyama a seed which he was to plant; when it sprouted, he was to jump over it one hundred times every day. As the seed grew and became a plant, Oyama later said, "I was able to jump between walls back and forth easily." The writer, Ikki Kajiwara, and the publisher of the comics based the story on the life experience Oyama spoke to them about – thus the title became "Karate Baka Ichidai" (Karate Fanatic).
In 1963, Oyama wrote What is Karate which became a best seller in the US and sold million copies all over the world. It is still considered by many to be the "Bible" of Karate to this day. It was translated into Hungarian, French, and English.
In 1945 after the war ended, Oyama left the aviation school. He began "Eiwa Karate Research Center" in Suginami ward but closed it quickly because "I soon realized that I was an unwanted Korean. Nobody would rent me a room." He finally found a place to live in Tokyo. This is where he met his future wife whose mother ran a dormitory for university students.
In 1946, Oyama enrolled in Waseda University School of Education to study sports science.
Wanting the best in instruction, he contacted the Shotokan dojo (Karate school) operated by Gigō Funakoshi, the second son of karate master and Shotokan founder Gichin Funakoshi. He became a student, and began his lifelong career in Karate. Feeling like a foreigner in a strange land, he remained isolated and trained in solitude.
Oyama attended Takushoku University in Tokyo and was accepted as a student at the dojo of Gichin Funakoshi. He trained with Funakoshi for two years, then studied Gōjū-ryū karate for several years with So Nei Chu (조영주 / 曺（曹）寧柱, 1908–1995), a senior student of the system's founder, Chojun Miyagi. So was a fellow Korean from Oyama's native province.
Korea had been officially annexed by Japan since 1910. During World War II (1939–1945) there was much unrest throughout Korea. As South Korea began to fight against North Korea over political ideology, Oyama became increasingly distressed. He recounts, "though I was born and bred in Korea, I had unconsciously made myself liberal; I felt repulsion against the strong feudal system of my fatherland, and that was one of the reasons which made me run away from home to Japan." He joined a Korean political organization in Japan to strive for the unification of Korea, but soon was being targeted and harassed by the Japanese police. He then consulted with Mr. So.
Around the time he also went around Tokyo getting in fights with the U.S. Military Police. He later reminisced those times in a television interview, "Itsumitemo Haran Banjyo" (Nihon Television), "I lost many friends during the war- the very morning of their departure as Kamikaze pilots, we had breakfast together and in the evening their seats were empty. After the war ended, I was angry- so I fought as many U.S. military as I could, until my portrait was all over the police station." At this time, Mr. So suggested that Oyama retreat to a lone mountain for solace to train his mind and body. He set out to spend three years on Mt. Minobu in Yamanashi Prefecture, Japan. Oyama built a shack on the side of the mountain. One of his students named Yashiro accompanied him, but after the rigors of this isolated training, with no modern conveniences, the student snuck away one night, and left Oyama alone. With only monthly visits from a friend in the town of Tateyama in Chiba Prefecture, the loneliness and harsh training became grueling. Oyama began to doubt his decision, so he sent a letter to the man who suggested the retreat. Mr. So replied with encouragement to remain, and suggested that he shave off one eyebrow so that he would not be tempted to come out of the mountain and let anyone see him that way. Oyama remained on the mountain for fourteen months, and returned to Tokyo a much stronger and fiercer Karateka.
Oyama gave great credit to reading The Book of Five Rings by Miyamoto Musashi, a famous Japanese swordsman, to change his life completely. He recounts this book as being his only reading material during his mountain training years.
He was forced to leave his mountain retreat after his sponsor had stopped supporting him. Months later, after he had won the Karate Section of Japanese National Martial Arts Championships, he was distraught that he had not reached his original goal to train in the mountains for three years, so he went into solitude again, this time on Mt. Kiyosumi in Chiba Prefecture, Japan and he trained there for 18 months.
In 1953 Oyama opened his own karate dojo, named Oyama Dojo (form of Gōjū-ryū), in Tokyo but continued to travel around Japan and the world giving martial arts demonstrations, including the fighting and killing of live bulls with his bare hands (sometimes grabbing them by the horn, and snapping the horn off). His dojo was first located outside in an empty lot but eventually moved into a ballet school in 1956. The senior instructors under him were T. Nakamura, K. Mizushima, E. Yasuda, M. ishibashi, and T. Minamimoto. Oyama's own curriculum soon developed a reputation as a tough, intense, hard-hitting but practical style which was finally named Kyokushinkai (Japan Karate-Do Kyokushinkai), which means 'the ultimate truth,' in a ceremony in 1957. He also developed a reputation for being 'rough' with his students, as the training sessions were grueling and students injuring themselves in practice fighting (kumite) was quite common. Along with practice fighting that distinguished Oyama's teaching style from other karate schools, emphasis on breaking objects such as boards, tiles, or bricks to measure one's offensive ability became Kyokushin's trademark. Oyama believed in the practical application of karate and declared that ignoring 'breaking practice is no more useful than a fruit tree that bears no fruit. As the reputation of the dojo grew students were attracted to come to train there from inside and outside Japan and the number of students grew. Many of the eventual senior leaders of today's various Kyokushin based organisations began training in the style during this time. In 1964 Oyama moved the dojo into the building that would from then on serve as the Kyokushin home dojo and world headquarters. In connection with this he also formally founded the 'International Karate Organization Kyokushin kaikan' (commonly abbreviated to IKO or IKOK) to organise the many schools that were by then teaching the kyokushin style.
In 1961 at the All-Japan Student Open Karate Championship, one of Oyama's students, Tadashi Nakamura, at 19 years old (1961)made his first tournament appearance, where he was placed first. Nakamura later became Mas Oyama's Chief Instructor as referenced in Mas Oyama's book, "This is Karate." In 1969, Oyama staged the first All-Japan Full Contact Karate Open Championships which took Japan by storm and Terutomo Yamazaki became the first champion, which have been held every year since. 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 formally establishing Kyokushin-kai, Oyama directed the organization through a period of expansion. Oyama and his staff of hand-picked instructors displayed great ability in marketing the style and gaining new members. Oyama would choose an instructor to open a dojo in another town or city in Japan, whereupon the instructor would move to that town, and, typically 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 a few students for his new dojo. 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 United States, Netherlands, England, Australia and Brazil to spread Kyokushin in the same way. Oyama also promoted Kyokushin by holding The All-Japan Full Contact Karate Open Championships every year and World Full Contact Karate Open Championships once every four years in which anyone could enter from any style.Kenji Kurosaki, taught many practitioners including Shigeru Oyama, Yasuhiko Oyama, Tadashi Nakamura, and Hideyuki Ashihara
Terutomo Yamazaki, the first champion of the All-Japan Full Contact Karate Open Championships and former professional kickboxer
Sonny Chiba, popular Japanese actor and martial artist
Hatsuo Royama, 9th dan, Kancho (Director) of the Kyokushin-kan International Honbu
Tadashi Nakamura, founder of World Seidō Karate Organization
Shigeru Oyama, Founder of World Oyama Karate
Yasuhiko Oyama, younger brother of Shigeru Oyama and cofounder of World Oyama Karate
Seiji Kanamura, longtime senior instructor and practitioner in USA
Bobby Lowe, 10th dan
Steve Arneil, 10th dan and founder of the International Federation of Karate (Kyokushin)
Jon Bluming, 10th dan and founder of the Kyokushin Budokai and IBK
Isobe Seiji, 8th dan, designated to Brazil, became Shihan after spread Kyokushin through South America, who taught and trained Francisco Filho, Glaube Feitosa and Everton Teixeira.
Hideyuki Ashihara, founder of Ashihara Karate
Yoshiji Soeno, founder of Shidōkan Karate
Yoshikazu Matsushima, 9th dan, President of IKO MATSUSHIMA, International Karate Organization Kyokushinkaikan
Katsuaki Satō, winner of the 1st World Full Contact Karate Open Championships and All-Japan Championships on two occasions
Jerry Bell, founder of Shotoshin-kai Karate
Jōkō Ninomiya, founder of Enshin Karate
Yousef Shirzad, 9th dan, founder of Kyokushin Karate in Iran, also founder of Shinzen Karate
Makoto Nakamura, winner of the 2nd and 3rd World Full Contact Karate Open Championships. The nickname is " King Of Kyokushin"
Keiji Sanpei, winner of three consecutive All-Japan Championships and 2nd-place finisher of the 2nd and 3rd World Open Championships
Shokei Matsui, 8th dan, Kancho (Director) of the International Karate Organization Kyokushin-kaikan (Matsui-Ha)
Francisco Filho, first non-Japanese karateka to win the World Full Contact Karate Open Championship in 1999
Nicholas Pettas, Last Uchi Deshi
Kenji Midori, winner of the 5th World Full Contact Karate Open Championships and current president of Shinkyokushinkai
Shehan Arshid Jan, 5th 'dan' and president of Pakistan region
Kenji Yamaki, founder of Yamaki Karate and winner of the 6th World Full Contact Karate Open Championships
Etsuko Shihomi, actress who specialized in action films
Oyama tested himself in a kumite, a progression of fights, each lasting two minutes, and each after the featured participant wins. Oyama devised the 100-man kumite which he went on to complete three times in a row over the course of three days.
He was also known for fighting bulls bare-handed. In his lifetime, he battled 52 bulls, three of which were purportedly killed instantly with one strike, earning him the nickname of "Godhand".
Oyama had many matches with professional wrestlers during his travels through the United States. Oyama said in the 1958 edition of his book What Is Karate that he had just three matches with professional wrestlers plus thirty exhibitions and nine television appearances.
In 1946, Oyama married a Japanese woman, Oyako Chiako (1926-2006) and had three children with her. In the late 1960s, Oyama and Chiako were having marital problems and decided to separate, and Chiako, who did not want her husband to start seeing other women, arranged for a Korean woman and family friend named Sun-ho Hong to become Oyama's companion for some time. With Hong, Oyama had three more children and he would remain romantically involved with both Hong and Chiako until the end of his life.
Later in life, Oyama suffered from osteoarthritis. Despite his illness, he never gave up training. He held demonstrations of his karate, which included breaking objects.
Oyama wrote over 80 books in Japanese and some were translated into other languages.
Before dying, Oyama built his Tokyo-based International Karate Organization, Kyokushinkaikan, into one of the world's foremost martial arts associations, with branches in more than 100 countries boasting over 12 million registered members. In Japan, books were written by and about him, feature-length films splashed his colorful life across the big screen, and comic books recounted his many adventures.
Oyama died at the age of 70, on Tokyo, Japan, April 26, 1994, of lung cancer, despite never being a smoker in any point in his life.
His widow Chiyako Oyama, made a trust foundation to honor his lifelong work.A manga about Oyama's legacy, Karate Baka Ichidai (literal title: "A Karate-Crazy Life"), was published in Weekly Shonen Magazine in 1971, written by Ikki Kajiwara with art by Jirō Tsunoda and Jōya Kagemaru. A 47-episode anime adaptation was released in 1973 which featured several changes to the plot, including the renaming of the Mas Oyama character to "Ken Asuka". A trilogy of live-action films based on the manga was also produced: Champion of Death (1975), Karate Bearfighter (1975), and Karate for Life (1977). The films featured Oyama's pupil, Japanese actor, and martial artist Sonny Chiba, in the main role. Oyama himself appeared in the first two films. Another film adaptation, Fighter in the Wind, was released in 2004, starring Yang Dong-geun.
During the 1970s, Oyama and some of his top students were featured in a documentary film The Strongest Karate (released as Fighting Black Kings in the US market) followed by two sequels, all having Ikki Kajiwara as executive producer.
Takuma Sakazaki (a.k.a. "Mr. Karate"), a character from SNK's King of Fighters and Art of Fighting video game franchises, was inspired by Mas Oyama. Within the mythology, Sakazaki is the founder and grandmaster of the fictional Kyokugenryu Karate, which is a nod to Oyama's own Kyokushinkaikan.
The works of manga author Keisuke Itagaki feature at least two characters inspired by Oyama: Doppo Orochi from Grappler Baki and Shozan Matsuo from Garouden.
The Pokémon Sawk was inspired by Mas Oyama.
The Kyokushin Way. ISBN 0-87040-460-1
What is Karate? ISBN 0-87040-147-5
This is Karate. ISBN 0-87040-254-4
Advanced Karate. ASIN B000BQYRBQ
Vital Karate. ISBN 2-901551-53-X
Essential Karate. ISBN 978-0-8069-8844-3