The main food source of O. gracilis is floating kelp and algae that they capture utilizing a waiting strategy in order to maintain cryptosis.

Like many crustaceans, the head and thorax combine into a cephalothorax and are completely covered in a continuous exoskeleton called a carapace.

The carapace is heart shaped, a brown, tan or grey color and 5 cm in length with four pairs of long thin walking legs (pereiopods).

Using fine hooked setae found on the carapace, the crab liberally decorates itself with algae, sponges, bryozoans and hydroids.

Its shell covered in sessile organisms makes the crab camouflaged to its environment.

During the mating process the male uses the first and second abdominal appendages to transfer sperm packets called spermatophores to the female.