Johann van Oosterzee Marais, civil servant and amateur ornithologist, was a son of Johan Charl Marais and his wife Augusta Wilhelmina Magdalena, born Duminy (and a great-grandson of Dr Johannes Knockers van Oosterzee*). He entered the civil service of the Cape Colony in April 1894 and from November 1896 was stationed at Knysna as a clerk attached to the Midlands Forestry Conservancy, Department of Agriculture. During his spare time he studied the birds of the region. He was an ardent collector, amassing large collections of bird skins. He was also a skilled and thorough taxidermist, and the quality and finish of his specimens were of the very highest order. In 1898 he spent some time at the South African Museum in Cape Town examining its study collection of birds. The next year he donated some 60 specimens from around Knysna to the museum, all of them rare and one representing a new species. He provided the type specimen of the new species to the museum in 1900. It was described by W.L. Sclater* in The Ibis in 1901 and named Laniarius maraisi (Marais' bush-shrike). In 1902 he sold a fine collections of over 400 bird skins, most of them collected in the Knysna district, to the Transvaal Museum (now the Ditsong National Museum of Natural History) in Pretoria.
In 1897 Marais published a paper on "South African birds arranged according to their diet" in the Agricultural Journal of the Cape of Good Hope (Vol. 10(9), pp. 527-530), in which he corrected a list published by Sclater earlier that year. In a subsequent article in the same journal, "Birds as friends or foes" (1897, Vol. 11(3), pp. 148-151) he discussed the classification of the weaver birds.
Marais was a member of the British Ornithologists' Union, and greatly interested in and in favour of the founding of the South African Ornithologists' Union, an event which took place a few months after his death. He tried to convince the Cape Government to protect certain species of birds that were valuable to farmers and, with the help of local authorities, was successful in some regions.
During the Anglo-Boer War (1899-1902) he served as a lieutenant in Kitchener's Horse, and on the staff of the 6th Corps, 2nd Brigade, Mounted Infantry. After the war he returned to his post at Knysna for a short period, but then left the civil service to undertake an extended collecting tour in the north of present Zimbabwe. He died of blackwater fever at Sipolilo(s), some 160 km north of Harare, aged 33. The birds that he collected during this tour were acquired by the Transvaal Museum.