Until 1921, Oskar Hamel was an officer and then senior officer in the Vienna Provincial Tax Office before turning to antiques and starting an independent business in 1923. He also collected coins and stamps. He joined the NSDAP in May 1933 and remained a member during the time it was banned. In 1937 he also joined the Vaterländische Front (Fatherland Front). Even before the annexation he worked with Karoline Nehammer in her antiques business at Piaristengasse 11 in Vienna's 8th district. In 1943 he registered his own sole proprietorship at the same address. In 1940 he became a court-sworn expert and in 1943 assessor in the art department of the Dorotheum. In 1942 he and Nehammer acquired Burg Seebenstein near Wiener Neustadt from the Prince of Liechtenstein and established a furniture depot in Palais Auersperg in Vienna. Hamel did business with the Special Representatives Hans Posse and Hermann Voss and was also a purchaser for Franz Josef II of Liechtenstein and his art consultant Gustav Wilhelm. In 1945 he appeared before the Vienna Volksgericht together with his business partner Karoline Nehammer and her brother Rudolf Prinz on suspicion of illegal enrichment (§ 6 of the War Criminals Act). Hamel was accused of having acquired artworks and valuables goods at auctions, which had been seized from removal goods, and to have profited from enforced sales by persecutees and in that way of having increased his assets during the Nazi period thirteenfold. Hamel died in custody before he was brought to trial and sentenced.

Provenance research regularly deals with objects acquired by museums from Hamel. For example, artworks belonging to Isidor and Jenny Mautner, which had been acquired in 1938 und 1939 through Oskar Hamel by the Städtische Sammlungen Wien, were restituted to the heirs. In other cases the original provenance has still to be clarified.