Carl-Oscar Avén trained at Carl Wilhelmson's painting school and at the Royal Academy of Fine Arts in Stockholm from 1915 to 1920. His works include crucifixes and altarpieces for Swedish churches, for example in Edsberg's church in Närke and Häggeby church in Uppland. He also modeled portraits in terracotta.

He is best known for his fountain sculpture. "Girl on a Shell" which he created on behalf of Näfverqvarns Bruk and another depicting a fish participated in the Stockholm Exhibition in 1930. He is strongly associated with Swedish modernism and the concept of Swedish Grace In Göta Lejon he decorated the gallery with muses and for the Norra Kungstornet he designed the building's sculptural elements (together with Eric Grate). He is represented at Moderna Museet and Waldemarsudde in Stockholm.