Oreoglanis majusculus, a species of the O. siamensis group sensu (Ng & Rainboth, 2001) is distinguished from all its congeners in having the posterior margin of the maxillary barbel villiform (vs. entire, crenulate or lobulate). It further differs from O. macropterus and O. insignis in having a caudal-peduncle depth of 6.1–6.4% SL (vs. 8.0–8.7 and 3.0–3.8, respectively); from O. setiger in having a caudal-peduncle length of 18.0–21.3% SL (vs. 26.1–28.8); from O. siamensis in having the posterior margin of the lower lip entire (vs. lobulated); from O. jingdongensis in having the caudal-fin lobes equal (vs. lower lobe longer); from O. immaculatus in having pectoral-fin rays 20 (vs. 17–18) and the lower-lip margin entire (vs. slightly indented); from O. heteropogon, O. laciniosus and O. sudarai in having posterior margin of lower lip entire (vs. lobulated); and from O. suraswadii in having posterior margin of lower lip entire (vs. with indistinct projections) (Ref. 86402).