Othmar Fritsch studied law for three semesters from 1906 at the University of Vienna after having completed a school-leaving course at a commercial college and the textile and weaving college in Vienna. In 1909/10 he lived in Berlin, London and Paris and took private lessons in the history of art. From 1911 he studied art history, philosophy and archaeology at the University of Graz and was awarded a doctorate there in 1914 with a thesis on Van Dyck's Italian sketchbook. He knew Egon Schiele and had his portrait painted by him in 1917. In that year he bargained with the painter on the price of the painting Schönbrunn Landscape. After being seriously wounded during the First World War, Fritsch was classed as a semi-invalid. He stated that he worked for a time as an assistant at the provincial painting gallery in Graz and also as an auctioneer. In 1928 he acquired a trade licence as a photographer and for a time between the wars he also had a share in Herzl Weinstube in Graz, which had originally been owned by his mother. Between 1930 and 1938 he lived not far from the city in Lannach Castle. According to a report after the Second World War by the Vienna police, Fritsch worked for many years before the war as an art buyer for the US media tycoon and collector William Randolph Hearst.
After the annexation of Austria to the German Reich, Fritsch moved to Vienna and opened an antiques dealership at Weihburggasse 9 in the 1st district. It had previously been a carpet shop owned by Emanuel Wertheim, who also dealt in antiques. Wertheim was persecuted as a Jew and was deported to Theresienstadt in 1942, where he perished in 1943. Fritsch moved into the apartment of the Jewish lawyer Richard Engländer and his wife Hedwig at Seilerstätte 5 in the 1st district and acquired Persian carpets belonging to Engländer, paying next to nothing for them. He acquired a gold tableware service from the Dorotheum thought to have belonged previously to the Rothschilds, along with several other objects, including a Torah ark belonging to the brothers Ludwig and Richard Herzog, which had been stored with a transport company and was later seized. Fritsch did not join the NSDAP but belonged to the National Socialist Motor Corps (NSKK) and – as an antiques dealer – to the Reich Chamber of Culture. Starting in March 1939, on behalf of the Property Transaction Office he liquidated twenty-one companies from various sectors belonging to Jews, particularly joineries and upholsterers. He was employed from 1943 as a non-permanent expert in the Dorotheum art department and in 1944 became a court-sworn expert. In 1947 the Volksgericht in Vienna instigated proceedings against him under section 6 of the War Criminals Act (Illegal enrichment). He contested the accusations, including the charge that the takeover of Wertheim's business and the Engländer apartment were "Aryanizations". The case was closed after several months. In 1965 Fritsch moved from Vienna to Bad Gastein.