The sheep nasal bot fly is a stout, grayish-brown fly covered with short hairs. Adult Oestrus ovis have black pits dorsally between the eyes on the frons and black tubercles among the yellow hairs on the yellow-brown scutum and scutellum. The abdomen is black with an irregular pattern of lighter marking which varies with the angle of illumination.
The sheep nasal bot fly, in common with other bot flies, have atrophied mouthparts The adults have a head and thorax with pits containing small bumps or tubercles.
Upon being deposited in the nostril of a sheep, the larva crawls onto the mucous membrane of the nasal passage, where it will remain for at least two weeks. Development of the 1st instar larva may be delayed, and individuals from the same larviposition may spend from 1 to 9 months in the 1st instar. This plays a role in the overwintering cycle.The pupal stage lasts 1-2 months depending upon temperature.
The female is ovoviviparous and matures about 500 eggs, which are deposited as newly hatched larvae in small batches of less than 50 at a time.When an opportunity presents itself, Oestrus ovis deposits larvae into the nasal cavity.