Born August 6, 1930, Ohshima began learning martial arts at the age of five. He studied sumo from the age of five until he was fifteen; kendo from eight to fifteen, and judo from nine to thirteen.
Ohshima began studying karate while a student at Waseda University, beginning in 1948, and became captain of the university's karate club in 1952. He trained under Shotokan's founder, Gichin Funakoshi, until 1953. Funakoshi personally awarded Ohshima his Sandan (3rd degree black belt) rank in 1952. In 1957 Ohshima received his Godan (fifth degree black belt), the highest rank awarded by Funakoshi. This remains the highest rank in SKA.
In 1952, Ohshima formalized the judging system used in modern karate tournaments. However, he cautions students that tournaments should not be viewed as an expression of true karate itself.
Ohshima left Japan in 1955 to continue his studies at UCLA. He led his first U.S. practice in 1956 and founded the first university karate club in the United States at Caltech in 1957. In 1959 he founded the Southern California Karate Association (SCKA), as additional Shotokan dojos opened. The organization was renamed Shotokan Karate of America in 1969.
Ohshima demonstrated the 19 Shotokan katas for Karate-Do Kyohan: The Master Text, Funakoshi's description of Shotokan. He also translated it into English in 1973, and published his own Notes on Training in 1998.
While still Shihan of SKA, Ohshima retired from Caltech in 1994. He currently lives in southern California.