In 1873 Oser was appointed primary physician in the newly opened hospital of the Israelite religious community, which he ran until his death. In the same year he completed his Habilitation. In Vienna, cholera broke out due to the large number of visitors to the 1873 World's Fair and the inadequate sewage system. So it was no coincidence that Oser did pioneering work in the field of cholera treatment. In 1874 he became a member of the Lower Austrian State Medical Council and from 1905 its chairman. In 1885 he became an associate professor of internal medicine at the Medical University of Vienna and in 1902 he received the professorship (o. Univ.-Prof.) with the title Ordinarius.