 After the war had ended Ostrowski moved to Göttingen where he wrote his doctoral dissertation and was influenced by Hilbert, Klein and Landau.In 1920, after having obtained his doctorate, Ostrowski moved to Hamburg where he worked as Hecke's assistant and finished his habilitation in 1922.In 1923 he returned to Göttingen, and in 1928 became Professor of Mathematics at Basel, until retirement in 1958.