Reiser was born in Vienna, son of a well known lawyer, Otmar Reiser sr. whose namesake father was a well-known mayor of
Maribor. He studied at the local gymnasium, at Novo Mesto (matriculating in 1882) and then joined as a reserve lieutenant in the 47th Infantry Regiment. He then went to the College of Agriculture in Vienna (graduating in 1887). He was involved in founding the
National Museum of Bosnia and Herzegovina in
Sarajevo and then joined its department of Natural History and worked there until retirement in 1919. He worked at the ornithological observatory at Ljubljana and studied Balkan birds. He also studied the birds of Sarajevo and contributed specimens to the Maribor Museum. He received an honorary doctorate from the University of Graz in 1932. His collections of eggs and bird skins were donated to the museum in Sarajevo. Reiser exhibited his collections at the 1891 International Ornithological Congress in Budapest, at the Millenium Exhibition, Budapest and at the international hunting exhibition in Vienna (1910). He took part in expeditions to Montenegro, Bulgaria, Greece, Brazil, and Serbia. He also collected plants and was in charge of the herbarium after the death of the botanist
Franz Fiala. In 1903 he took part in an expedition of the
Austrian Academy of Sciences to north-east
Brazil, during which he was the first to observe the
Spix's macaw in the wild since its original discovery 84 years earlier. == Works ==