He was born on 29 August 1850 in Vienna a son of the Viennese pharmacist Franz Zeidler. Othmar's brother, Franz Zeidler Jr. (1851–1901), also became a chemist and would collaborate with him on several projects. As a doctoral student with Adolf von Baeyer at the University of Strasbourg, then in Germany, Zeidler is credited with the first synthesis of the insecticide Dichloro Diphenyl Trichloroethane or DDT in 1874. Othmar returned to Austria before 1876 and, after working at the I. chemischen Universitätslaboratorium at the University of Vienna, became a pharmacist in the Fünfhaus district of the capital. He died in Mauer near Vienna on 17 June 1911.
^Though many sources mention 1859 as Zeidler's year of birth, this would make him a mere 14 years old at the time of his dissertation in 1873. The 1859 birth year was used by Joseph S. Fruton, in his Contrasts in scientific style: research groups in the chemical and biochemical sciences, Memoirs of the American Philosophical Society, Vol 191, 1990, page 373, as well as by Gerhard Oberkofler in Leopold Ruzicka, 1887-1976: schweizer Chemiker und Humanist aus Altösterreich - Page 44. The latter even has him quit his pharmacy studies at the Vienna University in 1872 (at the apparent age of 12/13) to move to Strassburg. However, Materialien zur Geschichte der Apotheken und Apotheker Niederösterreichs gives his year of birth as 1850; furthermore, his obituary at Oesterreichische Chemiker–Zeitung in 1911 states that Zeidler was 61 years at the time of his death, placing his birth around 1850 (in reality, Zeidler died 2 months shy of his 61st birthday).