Tanninen is well-known for her beloved Nunnu book series from the 1960s and its playful use of words and illustration. An animated TV series was also created based on the books. Her works have been published in Finland, Japan, England, Croatia, and Sweden. Her simple yet bold illustrations in graphic colors make her artwork feel as fresh today as when the first books were published in the 1960s.

Oili Tanninen lived abroad for 24 years due to her husband's work. The family first moved to Moscow in 1958 where her husband, Aarne Tanninen, worked as a Moscow correspondent for the newspaper Uusi Suomi. In later years, her family lived in London and Washington. Oili Tanninen lived in Helsinki, until her death in 2024.