It is found in humid forest from northeastern Honduras to western Ecuador, up to 1,800 m (5,900 ft) above sea level.

The fur, which is soft and dense, is usually dark brown above and light gray below; it is darker in juveniles.

Goldman considered the group to be similar to O. talamancae, but suggested that bombycinus and alleni might only be subspecies of O. nitidus.

[17] In 1939, Oliver Pearson added a third subspecies, O. b. orinus, from eastern Panama,[18] and in 1966 the species was first recorded from Colombia.

[19] Ronald Pine reviewed Oryzomys bombycinus in 1971, when 59 specimens of it were known, and first recorded the species from Nicaragua and Ecuador.

They considered O. bolivaris as probably the same as O. nitidus and listed castaneus as a synonym of O. capito (equivalent to modern Hylaeamys megacephalus and closely related species plus Transandinomys talamancae).

[23] Guy Musser and Marina Williams reviewed O. talamancae in 1985 and included O. castaneus as one of its synonyms, though without having examined the holotype.

[27] In 2006, Marcelo Weksler published a phylogenetic analysis of Oryzomyini ("rice rats"), the tribe to which Oryzomys is allocated, using morphological and DNA sequence data.

The two species are morphologically similar, but they could identify only one synapomorphy (shared-derived trait) for them: very long superciliary vibrissae (whiskers above the eyes).

[34] Musser and Carleton, writing in the 2005 third edition of Mammal Species of the World, used "Long-whiskered Oryzomys",[35] the 2009 IUCN Red List gave "Bolivar Rice Rat",[2] and Thomas Lee and colleagues used "Long-whiskered Trans-Andean Rice Rat" in 2010.

[10] In both species, the mystacial vibrissae (above the mouth) are also long and extend beyond the ears when laid back, but they are again much longer in T. bolivaris.

[52] Ungual tufts of long, white or gray hairs are present around the claws,[53] which are short and lack pigment.

[52] Although the subspecies previously recognized in "Oryzomys bombycinus" have been separated by small differences in skull features, Pine rejected these on the basis of his much larger samples.

The margins of the interorbital region contain prominent beads, which extend to the braincase as temporal ridges;[45] these are usually less well-developed in T. talamancae.

[58] Unlike in T. talamancae and H. alfaroi, the parietal bone is usually limited to the roof of the braincase and does not extend to the sides.

[41] The mandible (lower jaw) looks chunky and has a long condyloid process at its back;[41] that of T. talamancae is more slender.

[61] The capsular process, a projection at the back of the jaw which houses the root of the lower incisor, is poorly developed.

[61] As in this species, but unlike in many other rice rats, including H. alfaroi, the mesoflexus on the second upper molar, which separates the paracone (one of the main cusps) from the mesoloph (an accessory crest), is not divided in two by an enamel bridge.

[62] The hypoflexid on the second lower molar, the main valley between the cusps, is very long, extending more than halfway across the tooth; in this trait, it is similar to T. talamancae but unlike H. alfaroi.

[39] Its known distribution extends from northeastern Honduras, on the Caribbean seaboard, through eastern Nicaragua, Costa Rica, and Panama, into coastal western Colombia and northwestern and west-central Ecuador.

[39] The actual range of this species may be expected to extend further north and west, perhaps into Veracruz, southern Mexico, and western Venezuela, where it has not yet been recorded.

"[39] Two females with four embryos each have been caught in Panama in June, one with two in Nicaragua in September,[71] and one with four in Costa Rica in December.