Oka Masao (1898–1982) was a leading figure in the establishment of Japanese ethnology (cultural anthropology) since the 1930s and taught many of the next generation of ethnologists from Japan. He travelled to Vienna in 1929 to learn the methodology for studying the ethnogenesis of his own country, putting forward theories that questioned tennō-ideology of the time and became highly influential. During the war, he pushed for the establishment of an Ethnic Research Institute (Minken) to support the government in their ethnic policy in the occupied territories. Oka was also the founder of Japanese Studies at the University of Vienna in 1938. Despite these important—and at time controversial—roles, he is relatively unknown today. This article introduces recent scholarship on Oka’s life and legacy. It raises important questions about the role of ethnologists in politically sensitive times and counter-balances the Anglo-American narrative of the history of ethnology or social and cultural anthropology of Japan.