The infective stage of ostertagia is L3. The species within the subfamily Ostertaginae form a large and complex group, the taxonomy of which has not been fully elucidated, and is constantly under revision. O. ostertagi adults are slender reddish-brown worms. Adult males measure 6-8mm, adult females 8-11mm, and the eggs measure 70-86μm in length.
Identification of adult medium stomach worms is based on the structure of the bursa, genital cone, and spicules in males and on the dimensions of the oesophageal valve and the configuration of the synlophe in males and females. The cuticle in the anterior region is striated transversely whereas the rest of the body is unstriated and bears around 36 longitudinal ridges.