Olive Mary Hilliard (née Hillary) (4 July 1925 Durban) is a noted South African botanist and taxonomist.
Hilliard attended Natal University in the years 1943-47 where she obtained an MSc and later a PhD. She worked at the National Herbarium in Pretoria in 1947-48 and was a lecturer in botany at Natal University from 1954 to 1962. In 1963 she became curator of the herbarium at Natal University and a research fellow. Her special fields of interest were the flora of Natal and the taxonomy of Streptocarpus, Compositae and Scrophulariaceae.
In 1964 she formed a professional and personal collaboration with Brian Laurence Burtt (1913-2008) who played a large part in the revitalising of a moribund Royal Botanic Garden Edinburgh. Their collaboration resulted in numerous papers and three books, Streptocarpus: an African Plant Study (1971), The Botany of the Southern Natal Drakensberg (1987), and Dierama: The Hairbells of Africa (1991). 
Her collected specimens, mostly from the Natal Drakensberg and Malawi, number some 8 000 (of which 5 000 were collected with B. L. Burtt). She is commemorated in Plectranthus hilliardiae Codd, Schizoglossum hilliardiae Kupicha, Cymbopappus hilliardiae B.Nord., Agalmyla hilliardiae D.J.Middleton & S.M.Scott and Helichrysum hilliardiae Wild. This botanist is denoted by the author abbreviation Hilliard when citing a botanical name.