J.H. Overman studied agriculture in the Netherlands and at Halle, Germany, and then managed a farm in the Netherlands for a few years. On 1 October 1889 he was appointed as the first principal and main tutor of the School of Agriculture, Somerset East, which opened for tuition in February the next year. The school presented a two year course, but attracted only about a dozen students each year. None the less Overman conducted many small-scale experiments with crops, as part of the students' practical training, and in 1891 reported the first wheat breeding programme at an agricultural institution in southern Africa. Some lectures were given at the school by veterinary surgeon J.D. Borthwick*, the bee expert H.L. Attridge*, and agriculturalist A.C. McDonald*.
Overman contributed some short articles to the Agricultural Journal of the Cape of Good Hope during 1892, and in June 1893 addressed the Zwart Ruggens Farmers' Association (neighbourhood of Jansenville) on "The feeding of dairy cattle". At the request of individual farmers he often provided advice on farming matters, particularly those relating to dairying, and acted as a judge at various agricultural shows. In 1895 he made an official investigation of oyster cultivation at Saldanha Bay. In his report, "Oysters and oyster beds at Saldanha Bay", published in the Agricultural Journal (1895, Vol. 8(5), pp. 123-124) he recommended some experiments to test the profitability of the venture.
During 1895 Overman was away for five months on a trip to the Netherlands, where he made a study of agricultural training practices and visited breeders of Friesland cattle. Also during 1895 he became secretary of the Somerset East Agricultural Society. In 1896 he presented some plants to the Cape Government Herbarium. Three years later, during March-April 1899, he recommended to the government of the South African Republic (Transvaal) that a Department of Agriculture be established in that territory, and requested employment in such a department. Nothing came of the proposal, perhaps because of the outbreak of the Anglo-Boer War in October that year.
In 1906, after the untimely death of Professor F. Blersch*, Overman completed the manuscript of the latter's book and saw it through the press. The work was published under Blersch's name as Handbook of agriculture, with special reference to the requirements of South Africa. Overman was still principal of the School of Agriculture at this time.