Karyotypes of 7 taxa of the rodent genus Oligoryzomys trapped in 33 localities from an area ranging from 01°N to 32°S in Brazil were analyzed. Three species were trapped exclusively in the Cerrado biome: O. stramineus, diploid number (2n) = 52, fundamental number (herein, number of autosomal arms; FN) = 68; O. eliurus, 2n = 62, with 2 fundamental numbers, FN = 64 and 66; and Oligoryzomys sp., 2n = 70, FN = 74. In the Amazon, we caught O. cf. messorius, 2n = 56, FN = 58, and O. microtis, 2n = 66, FN = 74. Oligoryzomys nigripes (2n = 61, 62, FN = 80–82) was trapped in 24 localities in various biomes, and O. flavescens from several sites in southern Brazil presented the same karyotypes as those already described at other sites. The C- and NOR-banding were performed for all species, and the (T₂AG₃)_n telomeric probe hybridized in situ to both the short and long arms of all pairs of karyotypes of O. cf. messorius, O. eliurus, and Oligoryzomys sp. An analysis performed with 11 microsatellite DNA heterologous primers improved differentiation of O. nigripes from O. eliurus individuals, 2 species that presented 2n = 62 and that were trapped at the same site.