From 1902 to 1905 he taught Greek and ancient history at the universities of St Andrews and Edinburgh, and from 1905 to 1938 worked as a civil servant in the Board of Education.
Oppé was a distinguished collector of drawings, and monographs on Raphael and Botticelli, but subsequently concentrated on British art, particularly works on paper including those by William Hogarth, Alexander Cozens, John Robert Cozens, and Thomas Rowlandson, and wrote important catalogues on the English drawings in the Royal Collection at Windsor including those by Paul and Thomas Sandby.
[4] His collection of over 3,000 works of art on paper, including figurative drawings, portraits, and landscapes produced predominately between 1750 and 1850 was regarded as being of national importance and was acquired by Tate Gallery in 1996.
[5] The acquisition consisted of over 3,000 works of art on paper, including portraits, figurative drawings, and most notably landscapes from the ‘golden age’ of British watercolour painting (1750–1850).
[9] The archive comprises research notes, correspondence, annotated exhibition and auction catalogues, and other associated material compiled by Oppé throughout his career largely concerning eighteenth century British artists as well as an extensive set of diaries and notebooks that he maintained throughout his adult life.