Oocystis pusilla is a small green alga with ovoid or ellipsoidal cells, measuring 5 to 15 micrometers. Each cell has a smooth, firm cell wall and contains chloroplasts for photosynthesis. It typically occurs singly or in small colonies within a gelatinous matrix. Found in freshwater environments, it reproduces asexually by cell division and contributes to primary production, supporting aquatic food webs.