These fish live in fast-flowing streams in China, mainland Southeast Asia and the Indian subcontinent.

[3] They are easily distinguished from other catfishes by their strongly depressed head and body and greatly enlarged paired fins that have been modified to form an adhesive apparatus.

[1] The flattened shape of these fish and the large pectoral and pelvic fins provide essential adhesion in the fast-flowing waters they live in.

According to the original description of these groups, the O. siamensis species group is distinguished by having an emarginate caudal fin, and a lower lip notched medially with an entire or weakly laciniate posterior margin, while the O. delacouri species group is distinguished by having a lunate caudal fin and a lower lip without a median notch with prominent extensions along the posterior margin.

[1] It has been suggested that only the marginal morphology of lower lip can be employed to recognize the two species groups.