Emergent freshwater herb, attached to substrate, stolons absent; flowers bisexual. Leaves all basal, petiolate; lamina with veins prominent. Flowers of one or two kinds (see notes under O. ovalifolia), solitary at the apex of a stout stalk, subtended by united floral bracts; perianth of 2 whorls of 3 segments each, the inner whorl white or yellow, sometimes dark red-purple at base, larger than outer whorl; stamens 6–15; styles 3–15, each with a 2-lobed stigma. Fruit flask-shaped or ellipsoid, basal part of perianth persistent.
About 40 species in tropical and subtropical regions; 2 species occurring in Australia, 1 in Victoria.