An Oral arm is an anatomical

Anatomy

Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 structure of "true" sea jellies (or Scyphozoans), which belong to the class Scyphozoa

Scyphozoa

Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....

. Oral arms characterize Semaeostomeae

Semaeostomeae

Semaeostomeae is an order of large jellyfish characterized by four long, frilly oral arms flanking their quadrate mouths. The umbrella is domed with scalloped margins, and the gastrovascular system consists of four unbranched pouches radiating outwards from the central stomach; there is no ring...

, an order of large jellyfish.

The oral arms (of which there are usually four) are located around the mouth and hold the stinging cells, or cnidocysts, which are used to inject potential prey (or potential predators) with venom.