Leopold Oser was born in Mikulov (then Nikolsburg), Moravia, (now in the Czech Republic). In 1872 he became a lecturer in internal medicine at the University of Vienna and in 1885 was appointed assistant professor at the same faculty.
Oser wrote about stomach diseases for Eulenburg’s Enzyclopaedie der gesammten Heilkunde and about pancreas diseases in Professor Nothnagel’s Handbuch. He also researched and published articles about intestinal syphilis, intestinal stenosis and about the enlargement of the stomach. For many years Leopold Oser was chief of the general dispensary and physician at the Rothschild Hospital in Vienna.