Born in Tokyo, Onchi graduated from Keio University and joined the Toho studios.He debuted as a director with Wakai ōkami (1961), and first made a name for himself directing youth films such as Izu no odoriko (1967).After turning freelance, he also worked in Japanese television, serving for instance as the main director for Kizu darake no tenshi, an influential TV drama from the 1970s.
He won the award for Best Director at the 28th Hochi Film Award for Warabi no kō.