Morphological and morphometric comparisons of Otomys orestes dollmani, known only from Mount Gargues in the Mathews Range, central Kenya, indicate that the taxon is a valid species distinct from other African forms under which it has been previously synonymized (O. irroratus, O. orestes, O. tropicalis). Based also on these comparisons, the morphological recognition and distribution of O. orestes (including thomasi Osgood) are further clarified in relation to O. tropicalis (including elgonis Wroughton) in Kenyan mountains and O. typus proper in Ethiopian highlands; another taxon relegated to junior synonymy within O. typus, O. uzungwensis Lawrence and Loveridge from south-central Tanzania, is resurrected to species. Certain traditional characters used in Otomys taxonomy, in particular molar lamination, demonstrate conservative patterns of variation that complement spatial structure derived from morphometric analyses of craniometric data and that vindicate their continued utility in delimiting species. We argue that uncritical emphasis of polytypic species, applied following the biological species concept during the latter 1900s, has led to chronic underestimation of species diversity of Otomys confined to the Afromontane Biotic Region in eastern Africa, in particular those populations that inhabit afroalpine environments.