Oyama was born as Choi Young-Eui (최영의) in Gimje, South Korea, during Japanese occupation. At a young age he was sent to Manchuria to live on his sister's farm. Oyama began studying martial arts at age 9 from a Korean seasonal worker who was working on the farm. His name was Lee and Oyama said he was his very first teacher. The story of the young Oyama's life has been sensationalized in manga and movies so the line between fiction and fact has become obscure.
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, the then 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". 'Masutatsu' can also be pronounce 'baitatsu' in Japanese. Oyama was inspired to go to Japan by General Kanji Ishihara who was against the invasion of Asian neighbors (as a consequence, he was ostracized by higher ranks of the Japanese Army), to carve out his future in the heart of the Empire of Japan.
One story of Oyama's youth involves when Lee gave 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).
Oyama aspired to serve the Imperial Army during the war. He wrote a letter to the highest ranking officers with the blood from his fingers to apply for the Kamikaze pilot. Because it was the elite course he was rejected the first few times because of his back ground however, later Oyama recalls, "After the general saw I wrote in my own blood he knew I was ready to serve. The next week I was supposed to leave as Kamikaze, never returning to my home country." However, on the day of his mission, his airplane malfunctioned.
He later said in an interview for TV program," I had breakfast with my comrades ready to serve our country. In the evening when I returned for supper, the chairs were empty. There were no words to describe what I felt but I know I was given a chance to do something." One of the last TV programs Oyama taped was for Fuji Network (Japan) for a program called, 'Itsu Mitemo Haranbanjyo' (Always Stormy and Full of Drama) .
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 the "Bible of Karate" to this day. It was translated into Hungarian, French and English.