It includes eight species, two of which—the marsh rice rat (O. palustris) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions.

The name Oryzomys was established in 1857 by Spencer Fullerton Baird for the marsh rice rat and was soon applied to over a hundred species of American rodents.

The animals have broad feet with reduced or absent ungual tufts of hair around the claws and, in at least some species, with webbing between the toes.

[10] Within Oryzomyini, a 2006 phylogenetic analysis by Marcelo Weksler which used both morphological and DNA sequence data found some evidence that Oryzomys is most closely related to a group including Holochilus, Lundomys, and Pseudoryzomys.

Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3 gene alone instead placed Oryzomys among a group that included Nectomys, Sigmodontomys, and a few other genera.

[11] The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies (shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tufts of hairs surrounding the claws; having the back margin of the zygomatic plate of the skull at the same level as the front of the first upper molar; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette.

[14][15] Baird placed Oryzomys as a subgenus of the now-defunct genus Hesperomys and included only the marsh rice rat in it, a classification which was followed by Elliott Coues in 1874 and 1877.

[16] In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera.

[24] Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera.

[28] In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic, as species of Oryzomys were dispersed all over the oryzomyine tree.

[1] Merriam recognized 21 species within his group, but Goldman consolidated them into eight—the marsh rice rat in the United States, O. couesi in far southern Texas, Mexico, and Central America, and six others with small distributions.

[38] Hershkovitz described another species in the group, O. gorgasi from Colombia, in 1970[39] and the next year he noted that O. dimidiatus, previously classified as a Nectomys, was similar to O. palustris.

[38] While reviewing O. gorgasi in 2001, J. Sánchez H. and colleagues redefined and characterized the O. palustris group and listed O. couesi, O. dimidiatus, O. gorgasi, and the marsh rice rat as its members;[41] Guy Musser and Michael Carleton in the 2005 third edition of Mammal Species of the World additionally listed O. nelsoni from María Madre Island in western Mexico.

[43] The next year, Delton Hanson and colleagues published a study using DNA sequence data from the cytochrome b, interphotoreceptor retinoid-binding protein, and alcohol dehydrogenase 1 genes to assess relationships within Oryzomys.

He noted that the latter differed from the former in the generally darker, more brownish, longer fur, and larger sphenopalatine vacuities (openings in the mesopterygoid fossa, the gap behind the end of the palate).

[46] As Weksler's 2006 analysis included only O. couesi and the marsh rice rat among species of Oryzomys in the strict sense, he could not test those groups.

[36] Using morphological data, Voss and Weksler found a closer relationship between O. couesi and O. gorgasi to the exclusion of O. palustris, but with low confidence.

[44] In both species, the stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium.

[83] The front cusps of the first upper and lower molar (anterocone and anteroconid, respectively) are broad and not divided completely by an anteromedian flexus or flexid.

[85] The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character these two species share only with Oligoryzomys among oryzomyines examined.

[69] The oldest fossils date to the Rancholabrean of the United States, about 300,000 years ago; although there have been some earlier North American records, those are not in fact referable to Oryzomys or even Oryzomyini.

[69] Their extinction may have been caused by habitat destruction and by introduced species such as the small Asian mongoose and the brown and black rat.

[111] In contrast, the widespread species, the marsh rice rat and O. couesi, are common and of no conservation concern—indeed, both have been considered a pest—but some populations are threatened.

[113] The marsh rice rat is the natural reservoir of the Bayou virus, the second most common cause of hantavirus pulmonary syndrome in the United States.