Born in Tokyo in 1920 as the eldest son of Koshiro Onchi.
1937, he graduated from Meisei Gakuen Junior High School.
1941 Graduated from Tokyo School of Fine Arts (now Tokyo University of the Arts).
1942, he was a lecturer at Meisei Gakuen Junior High School.
1943 Meisei Gakuen full-time teacher.
In 1946 began magazine design.
In 1947 he started oil painting.
In 1949, he joined the Avant-Garde Art Association and left after about a year.
In 1950 he studied under composer Yukiaki Abe to learn harmony and composition.
In 1952 he wrote and staged a school play.
In 1957, he held his first solo exhibition at Ginza Yoseido. Held almost every year until 1963.
1965 Appointed principal of Meisei Gakuen High School.
1971 Published "Art and Education" (Daiichi Hoki Publishing).
1973 7th solo exhibition, since then held in parallel with small exhibitions.
1978 Again named principal of Meisei Gakuen High School, but resigned in 1980.
1982 Published "Onchi Koshiro Bookbinding Business".
1983, Third assignment as principal of Meisei Gakuen High School, resigned in 1987.
　　　　" Oyaji ganbare" Published by Kodansha.
1985 Tokyo Private School Education Research Institute, Advisory Committee, Chairperson of the International Education and Training Committee.
1986 Exhibited works of Koshiro Onchi at the Georges Pompidou Center (Paris) "Japan of Avant-Garde Art" and was invited to attend the opening ceremony.
1987 Established WKO Printmaking Institute. Engaged in research and production of woodblocks and printing for Koshiro prints.
　　　　He became an advisor to Meisei Gakuen and resigned in 1993.
1989 Lectured on the creative works of Koshiro Onchi at Harvard University's Yenching Research Institute. 　　　　
Exhibition of Koshiro Onchi's early abstract works - Focusing on "Tsukuhae" - was held at Galerie Tokyo Humanite.
1990 Held the "Kunio Onchi 70th Anniversary Exhibition" at the Ginza Art Center, and at the Kichijō Number One Hotel the " Onchi Koshiro Memorial Exhibition" .
1992 Edited "Abstract Expression: Koshiro Onchi Print Art Theory" and "The Mission of Binding: Koshiro Onchi Collection of Design Artists" (Abe Publishing).
1993 13th solo exhibition at Art Museum Ginza.
1997 Published "Souso Spring Snow" (Bunshin Publishing).
2001 Passed away at the age of 81.