Probolodus oyakawai is distinguished from its two congeners, P. heterostomus and P. sazimai, by the following characters: the base of the large epural not leaning against the urostyle bone (vs. the base of the large epural in contact, not fused the urostyle); first and second infraorbitals contacting but not overlapping (vs. relationship with overlapping or leaning against); posterior portion of the maxilla extended in relation to median portion (vs. posterior portion of the same width than median portion); anterior extension of nasal reaching the lateral projection of mesethmoid (vs. outpacing the end of the projection of mesethmoid). It further differs from P. heterostomus by having 41-43 perforated scales on the lateral line (vs. 45-56 perforated scales), 14-16 scale series around the caudal peduncle (vs. 18-21 scale series around the caudal peduncle), and body relatively shorter, with 33-34 total vertebrae (vs. 34-35 vertebrae) (Re. 96884).

Description: Dorsal-fin rays ii,9; anal-fin rays iv or v, 22-27; pectoral-fin rays i,10-i,12; pelvic-fin rays i,7 or i,7,i (Ref. 96884).