He was a keeper at the Botanical Museum 1900–1918, when he became professor of botany at the Royal Veterinary and Agricultural University.
[1] He was a member of the Royal Danish Academy of Sciences and Letters and served on the board of directors of the Carlsberg Foundation.
[2] Ostenfeld participated in the Ingolf expedition (1885-86) to the waters around Iceland and Greenland, and in 1911 in the International Phytogeographic Excursion to the British Isles.
The party studied the flora of parts of Ireland, including Killarney, Connemara and The Burren.
Also some geographical features in Greenland bear names that commemorate him: The French Academy of Sciences awarded him the Prix Desmazières for 1917.