Oocystis lacustris is a green algae characterized by its small, spherical or nearly spherical cells that may form colonies. Each cell has a smooth and delicate cell wall without prominent ornamentation. Within the cells, chloroplasts are typically disc-shaped and contain pyrenoids for photosynthesis, with a centrally located nucleus. Oocystis lacustris often aggregates to form colonies held together by a mucilaginous matrix. Reproduction can occur through cell division within colonies or the formation of zoospores. This algae species is commonly found in freshwater environments such as lakes, ponds, and slow-moving streams, where it thrives due to its specialized adaptations.