In 1971, Field Museum zoologist Philip Hershkovitz described a new species, Oryzomys gorgasi, on the basis of the single known specimen, an old male.

[6] The species was not recorded again until 2001, when Venezuelan zoologist J. Sánchez H. and coworkers reported on 11 specimens collected in coastal northwestern Venezuela in 1992, 700 km (430 mi) from the Colombian locality.

They referred the species to Oecomys,[9] a group of arboreal (tree-living), mainly South American rodents related to Oryzomys.

[16] Many aspects of the systematics of the O. couesi section remain unclear and it is likely that the current classification underestimates the true diversity of the group.

[19] Oryzomys gorgasi is a medium-sized oryzomyine[21] with small ears and large feet,[22] and is similar to the marsh rice rat in general appearance.

[22] At the base of the tail, the upper and lower sides differ in color and at the end is a short tuft of hairs.

The squamosal bone lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity,[25] a defining character of oryzomyines.

The back end of the lower incisor root is in a capsular process, a raising of the mandibular bone behind the molars.

[8] Within the species, the Colombian specimen differs from the Venezuelan animals in being larger in some measurements, but having smaller teeth, and in having oddly shaped wear facets of the incisors.

[31] The Colombian population is known from the holotype only, caught at Loma Teguerre (7°54'N, 77°W) in Antioquia Department, northwestern Colombia, near the Río Atrato, at about 1 m above sea level.

[2] In Venezuela, it was found on El Caimito, a small (57 ha, 140 acres) islet just east of the outlet of Lake Maracaibo in the state of Zulia,[32] where the only other native non-flying mammal is the opossum Marmosa robinsoni.

[33] El Caimito is separated from the mainland by a narrow, brackish channel and contains sand banks with xerophytic vegetation surrounded by marshy lagoons with Rhizospora mangle mangroves.

[8] Oryzomys gorgasi was caught in all habitats on the islet, but has not been found in other similar sites in northwestern Venezuela, where the introduced black rat is the only rodent collected.

[11] The species may be threatened by competition with introduced black rats[34] and destruction of its habitat,[35] but does occur in at least one protected area.

[33] The extinction of the Curaçao population may also have been caused by competition with the black rat, which has been found together with Oryzomys in subfossil deposits.