Oocystis is a planktonic genus of mostly freshwater green algae of the family Oocystaceae.

[3] Oocystis is solitary or borne in colonies of two, four or eight surrounded in a thin mucilaginous envelope or mother cell wall.

[1] Asexual reproduction occurs by the formation of two, four or eight autospores which are released by the rupture of the parental cell wall.

[1] However, other authors have stated that pyrenoids may be indistinct and difficult to observe,[5] and consider the two genera synonymous.

[4] As primary producers, Oocystis species are often preyed on by zooplankton,[8] or parasitized by Chytridium fungi.