Morphological variation within Eucalyptus series Subulatae subseries Oleaginae, a group of mallee taxa distributed in south-western Australia, was assessed by adult and seedling characteristics. A total of 36 adult morphological characters and 20 seedling characters was included in phenetic analyses of 82 individuals representing 29 populations covering all the taxa and the broad geographical distribution of the series. The phenetic analyses indicate relatively weak separation of all previously recognised taxa within E. subser. Oleaginae, except for E. longissima (previously referred to as E. grasbyi), which is distinctive in its seedling morphology, and to a lesser degree, E. ultima, which is also most easily distinguished by seedling morphology. E. eremicola, E. peeneri and E. sublucida are not well differentiated from one another, differing significantly only in adult leaf colour and gloss. The three previously recognised species of the oil mallee complex (E. kochii, E. plenissima and E. ‘horistes’) are also only weakly differentiated from one another. Two variants within E. kochii were observed to differ from each other in several adult vegetative and floral characteristics and are recognised at the subspecific level. A new taxonomy for E. subser. Oleaginae is presented, based on phenetic analyses combined with extensive field, glasshouse and herbarium examination of all taxa in the subseries. A new species is described (E. longissima Nicolle sp. nov.) to accommodate populations of mallees previously erroneously referred to E. grasbyi. New combinations are made for E. peeneri [E. eremicola subsp. peeneri (Blakely) Nicolle comb. nov.] and E. oleosa var. borealis [E. kochii subsp. borealis (C. Gardner) Nicolle comb. nov.] and two new subspecies of E. kochii are described (subspp. amaryssia Nicolle and yellowdinensis Nicolle). Eucalyptus sublucida is considered to be synonymous with E. eremicola subsp. peeneri. Keys to the taxa within the subseries are presented.