Oton Postružnik was born 26 March 1900 in Maribor, then in Austria-Hungary, today in Slovenia. His childhood was spent in Pregrada, where he completed his primary school education in 1910. Following that, he began high school in Krapina, but in 1913 the family moved to Zagreb. There, in 1915, Postružnik enrolled at the private art school of Ljubo Babić. In 1917 he participated in an anti-Hungarian demonstration, where he gave a speech, and received a warning from the authorities.

In 1918, Postružnik completed his high school education and enrolled at the College of Art in Zagreb. By 1920, he left to spend some time in Prague, returning in 1922 to continue his studies at the newly founded Academy of Fine Arts. He studied drawing with Maximilian Vanka, painting with Ljubo Babić, and graphics with Tomislav Krizman. He was also one of the first students to study ceramic techniques with Hinko Juhn.

Postružnik opened his own private art school, along with the painter Ivan Tabaković, which they ran until the end of the 1920s. In 1924, he received a royal government scholarship and spent some time studying in Paris. In 1927 he graduated from the Academy in Zagreb.

Postružnik was one of the founders of the Earth Group (Zemlja), an artists’ association that was established in early 1929. He participated in all their exhibitions until he left the group in 1933. He received a scholarship from the French government in 1935, and went to Paris for a second time.

In 1937 he organized his first solo exhibition in the Salon Ulrich in Zagreb. In 1950 he was elected professor at the Academy of Fine Arts in Zagreb, where he worked until 1970.

He received the Vladimir Nazor Award for lifetime achievement in the arts in 1964.

In 2000, the Croatian Postal Service issued a stamp with Postružnik’s “Klek”, 1929, as part of its Croatian Modern Art series.