Van Os was appointed professor of Art and Cultural History at the University of Groningen in 1974, and subsequently became dean of the Faculty of Arts in 1984. From 1989 to 1996, Van Os was general director of the Rijksmuseum. He organised numerous exhibitions, such as ‘Prayer Embedded in Beauty’ (Gebed in Schoonheid). In 1996, he stepped down as director of the Rijksmuseum and was appointed University Professor holding the chair in Art and Society at the University of Amsterdam (UvA). Van Os regularly gives guest lectures at various foreign Universities, including Smith College Northampton in America, the Zentralinstitut für Kunstgeschichte (‘Central Institute for Art History’) in Munich and the Harvard Centre for Renaisscance Studies in Florence.
Van Os gained nationwide renown in the Netherlands for publications, lectures and interviews, and especially as presenter of television programmes about art, such as Museumschatten (‘Museum treasures’) and Beeldenstorm (‘Iconoclasm’). In recent years, he has organised exhibitions in the Groninger Museum, de Nieuwe Kerk (‘the New Church’) in Amsterdam, Museum Catharijnecovent in Utrecht, the Mauritshuis in The Hague, and the Hermitage Amsterdam. Van Os is a member of the International Board of Trustees Hermitage St. Petersburg, and sits on the Board of Trustees of the European Fine Art Foundation. He is also chairman of the jury for the P.C. Hooftprijs for Essays, chairman of the Vetting Committees for TEFAF Maastricht and honorary member of various scholarly academies in the Netherlands and abroad.