Onaga Tamotsu was born in Himi City, Toyama prefecture, and studied lacquer under Yamazaki Ritsuzan (1895-1969). In 1951 he began exhibiting at a national level with Nitten (the Japan Fine Art Exhibition). In 1964 his entry to the Nitten won the Tokusen (Nitten Speciality Prize) as well as the Hokuto award. The following year he was awarded mukansa (non-vetted status). During his career he also assisted as a judge for the Toyama Prefectural exhibition.

Onaga participated in numerous international exhibitions including the 1965 Berlin Fine Art Festival and in the same year he was honoured with a membership to the Gendai Kōgei Bijutsuka Kyōkai (Association of Contemporary Craft Artists). In 1966 he exhibited in Rome with the Nihon Gendai Kōgei Ten (Japan Contemporary Art Craft Exhibition) and again in London the following year.

In 1968 his work for the Gendai Bijutsu ten (Exhibition of Contemporary Fine Art) won both the Member’s ward and the Minister of Culture Award. His work also won the Kikka sho (Chrysanthemum Award) at the 1969 Nitten. From 1970 until 1976 Onaga continued to exhibit at the Nitten and the Gendai Kōgei Bijutsu Ten (Modern craft and art exhibition). In 1976 he received a commission from His Highness the Emperor, one of Japan’s greatest honours.