Oligoryzomys nigripes is a species that has been further divided into different sister taxa throughout history.

Two other species, Oligoryzomys delticola (also known as the delta pygmy rice rat[5] or the large colilargo)[2] and Oligoryzomys eliurus (also known as the Brazilian pygmy rice rat[6] or the Brazilian colilargo)[2] have been recognized as synonyms of O. nigripes since 2005; they cannot be distinguished on the basis of morphology, karyotype, or morphometrics.Oligoryzomys nigripes is found from Pernambuco in northeastern Brazil through the Atlantic Forest and Cerrado into Paraguay, Uruguay, and Argentina, where it occurs in the provinces of Chaco, Misiones, and Buenos Aires.

[3] The pelage colors for this species resemble brown, dark yellow, and grey.

[4] The totaled head plus body length of the model species for O. nigripes is 9 cm.

[9] They scavenge for nutrient sources in things such as fruits, insects, and seeds.