Oesia ranged in length from 2.4mm – 120mm, with an average length of 53mm. The anteriormost region is an oval or “acorn” shaped proboscis, which gives the acorn worms (Enteropneusta) their common name. The proboscis is frequently preserved with a darker, more dense carbon content than surrounding tissues, suggesting that it was highly muscular as the proboscis is in modern acorn worms. Behind the proboscis was a short cylindrical region called the collar, which enclosed the mouth. Behind the collar was a long, cylindrical region called the trunk, which maintain a roughly even width throughout. Unlike in modern acorn worms or its contemporary Spartobranchus, the trunk of Oesia was not divided into a pharynx and a posterior trunk. Instead, the collagenous gill bars that define the pharynx continue throughout the entire length. This gives Oesia a relatively inflexible appearance. At the posterior end of Oesia was a bilobed shaped attachment structure. The interior of this structure also preserved highly concentrated carbon relative to surrounding tissue which, along with its shape, suggests that this appendage was for grasping. Oesia is often found inside another fossil previously described as the alga Margaretia dorus, but which is now recognized as the secreted dwelling of Oesia, which was likely used for feeding as a pre-filtration device. This tube was typically twice the width of Oesia and could reach nearly 50 cm in total length. The tube contained a series of spirally arranged pores and could bifurcate into branches as many as 5 or 6 times (although 1 or 2 bifurcations is more common).