Traits of O. hammondi that supported the latter placement include: a relatively short palate that does not extend behind the maxillary bones; simple posterolateral palatal pits; absence of a capsular process (a raising of the bone of the mandible, or lower jaw, at the back end of the incisor); and presence of the posteroloph on the upper third molar (a crest at the back of the tooth). In these characters, O. hammondi differs from many or most Oryzomyini and is similar to some species outside Oryzomyini, but all traits of O. hammondi are present in at least one other member of the tribe.Weksler, 2006, pp. 124, 34–36, 41–42, 48 Traits shared by O. hammondi and Oecomys included: tail with the same coloration above and below (unicolored); parietal bones extending to the sides of the skull; narrow zygomatic plate, without a zygomatic notch; posteroloph present on upper third molar; mesoflexus (a valley in the molar crown in front of the mesoloph crest) on upper second molar not divided in two.Weksler, 2006, pp. 138, 30–32
