Professor; museum director. Van Os studied history and art history at Groningen University between 1957 and 1964. From this university he received his doctor's degree in 1969. His dissertation dealt with iconological problems in Sienese painting between 1300 and 1450. In 1974 he obtained the position of professor of the history of art and culture at Groningen University. From 1984 until 1989 he served as the Dean of the faculty of Letters. In 1989 he was appointed general director of the Amsterdam Rijksmuseum, a position he held until 1996.