In Grade 4, started playing baseball for the Nagoya Hirabari Junior Baseball club. From middle school, Ōshima played for Tōkai Challenger in the junior boys league as both pitcher and outfielder.
In high school of his third year, Oshima batted lead-off and pitched for Kyōei High School in the summer.[2]
After entering Komazawa University. Oshima consolidated as an outfielder. In the Eastern University League of Tokyo, Oshima hit over .300 for 3 consecutive seasons from fall of 2006 including capturing batting title honours in fall of 2007 with a .395 average.[3] Through his university career, Oshima played 83 games, hit .290 with 1 homerun and 20 RBIs. He was selected in the Best 9 twice.
After graduating university, Oshima joined Nihon Seimei in the industrial league. In his first year he was installed as lead-off hitter and in January 2008, batted .563 in the Japanese National Championships. He was selected in the industrial league Best 9 in the same year.
^Kubota, Tatsuo (2018-08-06). "「えっ、あの人が？」 高校時代に投手だった"意外"なプロ野球選手" ["Really, that guy?" Professional players that were actually pitchers in high school]. Asahi Shimbun (in Japanese). Retrieved 2020-02-24.
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