Henry Miller Oakley came to the Cape Colony from Taunton, England, and married Frances Geddes Bain (daughter of Thomas C.J. Bain*) with whom he had six children. In November 1880 he entered the civil service in the Railway Department and three years later was given a fixed appointment as a third class clerk. In October 1887 he became registrar of gold mines at Knysna; then issuer of process at Millwood, a gold mining centre near Knysna (October 1888); and later acting inspector of mines at Knysna and special justice of the peace at Millwood (March 1891). The Report of the Inspectors of gold mines for the year 1891 (Parliamentary Report G28 '92) was written jointly by Oakley and P. Fletcher*. A few months after his latest appointment, in June 1891, he was instructed to investigate the discovery of gold in the Prince Albert district and the next month was appointed inspector and registrar of mines at Prince Albert. The brief report of his investigation was included in the Report of the Geological and Irrigation Surveyor upon his recent investigation into the gold discoveries upon the farms Spreeufontein and Klein Waterval in the district of Prince Albert (A12 '91), by Thomas C.J. Bain*. His report was also published in a pamphlet, The nuggets of the Gouph and the Prince Albert gold fields, with map and sections... (London, 1891).
In June 1892 Oakley's post at Prince Albert was abolished and he was transferred to the Department of Agriculture in Cape Town as a first class clerk (principal clerk from November 1897), a post he occupied until he left the civil service in 1899. By 1903 he was a member of the South African Association for the Advancement of Science and was still a member in 1906. At that time he became a collector of insects for the South African Museum, Cape Town, for a few years. For example, in 1906 he presented the museum with various insects from Kenhardt in the Northern Cape, some of them very rare, as well as some rare ones from Darling. The next year he and J.M. Bain* presented a collection of different orders of insects from British Bechuanaland (now part of the Northern Cape), many of them little known species and including four species as yet undescribed. In 1909 Oakley sent in more insects from the same region, as well as two live land monitors (Varanus albigularis).