Oithona is a planktonic crustacean genus found in marine, brackish, fresh water environments.

[1] Oithona has been described as the most ubiquitous and abundant copepod in the world's oceans.

[2] It was first described by Baird in 1843 using the species Oithona plumifera as taxon type.

[3] The following species are recognized:[1]