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He attended classes on natural history and geography at the University of Vienna, and in 1885 was part of an Austrian exploratory expedition of the Congo Basin headed by Oskar Lenz. However, he had to leave the expedition early due to illness. In 1886 he did ethnographical research on the island of Fernando Po. When he returned to Europe, he obtained his PhD from the University of Leipzig (1888).
Baumann is best known for his exploration of the interior of German East Africa (present-day Tanzania, Rwanda and Burundi), and producing maps of the region. In 1888 he explored the Usambara region with geographer Hans Meyer, with designs of continuing on to Mount Kilimanjaro. However, their progress was stopped due to ramifications associated with the so-called "Abushiri Revolt" (1888–89). Within a matter of days Baumann and Meyer were captured and held as prisoners. According to the British Rear-Admiral Edmund Fremantle: 'They had been robbed, stripped, and subjected to every indignity, but except that they have lost all, including their instruments and observations, they do not seem much the worse.' After a large ransom of 2,000 rupees was paid to rebel leader Abushiri ibn Salim al-Harthi were the two men released.
Baumann's most celebrated mission was the 200-member "Maasai Expedition" of 1891-1893. On the expedition, he performed map-making duties, and was the first European to visit Lake Eyasi, Lake Manyara and Ngorongoro Crater. As a result of the journey, he produced a book titled "Durch Massailand zur Nilquelle" (By Massailand to the Source of Nile) in 1894. In 1892, he became the first European to enter Rwanda.
In 1896, Baumann was appointed consul to Zanzibar by the Austro-Hungarian government. However, he died a few years later of an infectious disease at the age of 35. Today at the Vienna Museum of Ethnology are nearly 3500 artifacts that Baumann amassed from his African journeys. These include weapons, tools, jewellery, herbal perfumes and musical instruments.
Written works by Oscar Baumann
- Beiträge zur Ethnologie des Kongo (Contributions to the ethnology of the Congo), Vienna (1887)
- Fernando Po and the Bube. Vienna (1888)
- In Deutsch-Ostafrika während des Aufstandes (In German East Africa during the Rebellion), Vienna (1890)
- Usambara (Usambara), Berlin (1891)
- Karte des nordöstlichen Deutsch-Ostafrika (Map of northeast German East Africa), Berlin (1893)
- Durch Massailand zur Nilquelle (By Massailand to the Source of the Nile), Berlin (1894)
- Die kartographischen Ergebnisse der Massai-Expedition (The cartographic results of the Massai expedition), In: Petermann's Geographische Mitteilungen, Ergebnisheft 111, Gotha (1894)
- The Sansibar archipelago. 3 booklets. Leipzig (1896–99)
- Afrikanische Skizzen (African sketches), Berlin (1900)
- Maps and Expedition Information on German East Africa Dr. Oscar Baumann
-  Across East African Glaciers By Hans Meyer, E.H.S. Calder
- Parts of this article are based on a translation of the equivalent article at the German Wikipedia, who sources include ADB:Baumann, Oskar @ Allgemeine Deutsche Biographie and Baumann, Oskar @ NDB/ADB Deutsche Biographie.
- Fremantle to Admiralty, 23 October 1888, FO 403/107, TNA