Oosumi was born in Hyougo Prefecture, Japan, and graduated from School of Media Science at the Tokyo University of Technology. He got his start in the entertainment industry as the leader of a puppet theater in Kobe. This led to an association with Tokyo Movie Shinsha, one of the first Japanese animation studios.

In 1969, Oosumi collaborated with animator Yasuo Ootsuka in the Moomin TV series, which was an immediate hit. But Finnish author Tove Jansson, creator of the Moomin books, objected strongly to the depiction of the characters in the series. According to Jansson, "My Moomin is No car, No fight, and No money." The series was shifted to another studio after 26 episodes.

Also in 1969, he collaborated with Ootsuka on the classic Lupin III TV series. This series opened to poor ratings and was cancelled after only 23 episodes. Oosumi directed the first seven episodes and episodes nine and twelve, with the remaining episodes being directed jointly by Hayao Miyazaki and Isao Takahata. Oosumi was fired by the studio for refusing to adapt the sophisticated series for a children's audience. He went on to direct film and television animations for a number of other studios, and for the national broadcaster NHK. In 1993, he returned to direct one more Lupin episode, entitled Orders to Assassinate Lupin.

(Source: Wikipedia)