DNA Diagnosis. This species can be distinguished from O. ferreirai by combinations of the following nucleotide characters: COII region: 528T, 561C, 567C, 582A, 585G, 603C, 606C, 612T, 618C, 648G. Lysine tRNA region: 22T, 41C, 45G. ATP8 region:6A, 12C, 15T, 27G, 30A, 33G, 37T, 54G, 72C, 84C; 112A.ATP6 region: 87C, 96C, 111C, 114C, 138A, 150T, 192G, 198C, 213T, 217T, 219T, 226G, 249C, 258G, 265T, 267A, 282C, 285A, 294G, 300G, 312G, 313T, 318G, 324A, 330G, 333C, 339T, 351T, 363A, 393T, 394G, 410C, 429C, 447C, 448C, 450C, 474G, 489T, 495C, 508T, 528C, 552T, 576G, 583A, 585T, 586T, 591T, 600A, 607C, 625G, 627T, 633A, 654G, 657A.

Morphological features (data from Kanazawa, 1966). Tip of snout to anus 45.5 to 53.2; snout to origin of dorsal fin 55.9 to 61.9; snout length 3.7 to 6.5; eye diameter 3.1 to 8.7; head length 19.6 to 26.9; body depth 14.6 to 20.7. These data are given in percentages with respect to standard length. Branched dorsal fin rays 42 to 50; unbranched anal fin rays 49 to 58; pectoral rays i, 6; scales along the lateral line to base of caudal fin 30 to 37; pre-dorsal scales 16 to 19. Adult color is varied and can be silver, yellow, or dark greenish, paler ventrally.

Distribution. Essequibo River basin; Amazon River basin not including the Negro River basin except its Branco River affluent.