Ohshima began studying karate in 1948, while a student at Waseda University. There, he trained under Shotokan's founder, Gichin Funakoshi, and later became captain of the university's karate club, in 1952. That same year, Funakoshi personally awarded Ohshima the rank of 3rd dan black belt.Ohshima's seniors at the club also influenced his development in karate; they included Shigeru Egami, Toshio Kamata-Watanabe, Hiroshi Noguchi, Tadao Okuyama, and Matsuo Shibuya. It was also during this period that he befriended Mitsusuke Harada, who would go on to become a pioneer of karate in Brazil and the United Kingdom. Oshima was a member of the Japan Karate Association in its earliest days.