After the war 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.
Following the war's conclusion, Ostrowski relocated to Göttingen, where he wrote his doctoral dissertation and was inspired by Hilbert, Klein, and Landau. In 1920, after earning his doctorate, Ostrowski moved to Hamburg, taking on Hecke's assistant role and completing his habilitation in 1922.