Ōshima's first World Tour tournament was the Polish Open 2012, where he competed in the U21 category, and in December he rose to number 203 in the world rankings . At the Korea Open 2014, he made it to the quarter-finals, jumping 65 places to 137th in July. Together with his doubles partner Masataka Morizono, he won the Czech Open and with it his first World Tour medal, in September 2014 he was in the top 100 for the first time. At the 2015 World Cup , Ōshima and Morizono made it to the quarter-finals in doubles, where they later won Winners Xu Xin and Zhang Jike lost in the decisive set. Ōshima won bronze in doubles and mixed at the Asian Championships, and his first individual medal at the Philippines Open with gold. In September 2015, he jumped from 60th to 22nd in the world rankings, and he also qualified in singles, doubles and U21s for the Grand Finals , where he won gold in doubles and U21s, and then a new one with 18th place in the world personal record reached. In 2016 he won the silver medal at the Team World Cup in Malaysia , further successes in doubles - including silver at the Grand Finals - and in singles shima's victory at the Swedish Open. At the 2017 World Cup , he and Morizono won the silver medal in doubles, and gold at the Grand Finals at the end of the year. At the 2018 World Cup , he was part of the Japanese team that started as the reigning runner-up, but surprisingly lost to England in the group stage and South Korea in the quarter-finals and was eliminated without a medal. In doubles with Morizono, Ōshima won the Austrian Open 2018 and qualified for the fourth time in a row for the Grand Finals, in which they were eliminated in the semifinals against Ho Kwan Kit / Wong Chun Ting .
Japan