The Mount Oku rat is a medium-sized species growing to a head-and-body length of about . The dorsal fur is glossy and soft, dark brownish-black tinged with russet, with no stripe along the spine. The individual hairs are dark grey, banded with ochre and with black tips, and there are numerous longer, black guard hairs. The underparts are yellowish-grey, the hairs having grey bases and yellowish tips. The limbs are yellowish-brown, with four digits on the front feet and five digits on the hind. The tail is about the same length as the body, and is covered with scales and small black bristles.