Oligosarcus amome can be distinguished from all congeners except O. platensis and O. itau by having two conspicuous series of teeth on the premaxilla. It differs from O. platensis in having premaxillary teeth in the outer series pentacuspidate (vs. tricuspidate); premaxillary inner series with five teeth penta to heptacuspidate (vs. bi to pentacuspidate); ten to eleven maxillary teeth (uni to pentacuspidate) (vs. vs. 17 teeth (uni to tricuspidate); dentary teeth uni to heptacuspidate (vs. uni to pentacuspidate); 20-23 branched anal-fin rays (vs. 27); 39-41 lateral line scales (vs. 44); and 14-16 predorsal scales (vs. 24).

Oligosarcus amome can be diagnosed from O. itau in having premaxillary teeth in the outer series pentacuspidate (vs. tricuspidate); 39-41(vs. 41-42) scales on lateral line, and 14-16 (vs. 16-17) predorsal scales. It can be further distinguished from O. itau by following morphometric characters: predorsal distance (51.4-53.5 vs. 54.7-57.0% SL); body depth (32.0-34.0 vs. 36.3-37.5% SL); pectoral-fin length (18.6-20.7 vs. 22.7-23.9% SL); pelvic-fin length (14.5-16.2 vs. 18.2-20.0% SL); pectoral to pelvic-fin distance (24.1-25.7 vs. 19.7-20.1% SL); head length (26.7-27.8 vs. 28.7-30.8% SL); peduncle depth (10.5-11.4 vs. 12.6-13.2% SL); peduncle length (10.5-12.0 vs. 14.9-16.0% SL); eye diameter (26.9-29.3vs. 31.6-33.2% HL); and snout length (27.5-29.9 vs. 23.9-24.6% HL). In addition to the presence of two series of teeth on the premaxilla, Oligosarcus amome is distinguished from O. brevioris, O. jacuiensis, O. jenynsii, and O. oligolepis, occurring in the río Uruguay basin, by several characters. It is distinguished from O. brevioris, O. jacuiensis, and O. jenynsii by having 17-18 circumpeduncular scales (vs. 21-23, 21-25, and 12-16 respectively), 39-41 lateral line scales (vs. 47-56, 55-65, and 54-63 respectively), and 3 ectopterygoid teeth (vs. 7-18, 10-20, and 12-16, respectively). It differs clearly from O. oligolepis by having 17-18 circumpeduncular scales (vs. 25-27), 39-41 lateral line scales (vs. 72-79), and 3 ectopterygoid teeth (vs. 14-16). It is distinguished from O. pintoi in having 20-23 (vs. 24-28) branched anal fin-rays; 10-11 (vs. 15-23) maxillary teeth; 3 (vs. 5-14) ectopterygoid teeth; and penta to heptacuspidate (vs. all tricuspidate premaxillary teeth) (Ref. 99820).