O. ornatus was first described by Alfred Philpott in 1927[2] using a male specimen he collected in Golden Downs on 8 January 1926.

[2] George Vernon Hudson described and illustrated the species under the same name in 1939.

[5] In 1975 David Edward Gaskin assigned Crambus ornatus to the genus Orocrambus.

Forewings, costa moderately arched, apex blunt-pointed, termen rounded, oblique; brassy brown to chocolate brown; markings white; a basal patch on costa half enclosing a brown area; a broad irregular band at 1⁄4, not reaching dorsum, outwardly strongly dentate; on fold before this a large spot of mingled blackish and white scales; an elongate black mark about middle of wing at 1⁄3; a crescentic white area sprinkled with brown on costal half from about 1⁄3 to 4⁄5, enclosing an elongate spot of blackish-brown on costal margin; beneath this costal spot a prominent ring of brown enclosing a white area with a central brown dot; second line pure white, dentate, preceded on costa by a small blackish-brown dot and followed by a much larger one; a white area beneath the latter reaching apex; fringes fuscous, irregularly barred with white.

[8] This species has the "Nationally Critical" conservation status under the New Zealand Threat Classification System.