Other flowering plants

Ochnaceae

Ochnaceae DC. (including Medusagynaceae and Quiinaceae) is a conserved pantropical family of Malpighiales: about 500-550 species, 30-37 genera of evergreen trees and shrubs with stipules, yellow petals, and superior ovaries.

Ochroma

Ochroma (balsa tree, Ochroma pyramidale) is a monotypic Malvaceae genus native from S. Mexico to tropical America. Its wood, 60–380 kg/m³, is the lightest commercial timber, used in sandwich panels; Ecuador supplies 80–90% of trade.

Ocimum

Ocimum (basil genus), Lamiaceae mint family, pantropical aromatic herbs and shrubs named by Linnaeus in 1753; about 65 species mostly African, including sweet basil (O. basilicum) and holy basil (O. tenuiflorum), valued for terpenoid and phenylpropanoid essential oils.

Ocimum gratissimum

Ocimum gratissimum (clove basil, African basil, ram/Vana tulsi) is a Lamiaceae aromatic shrub of the Old World tropics, grown for eugenol- or thymol-rich essential oil with multiple chemotypes, used in food, medicine, ritual, and as an insect repellent.

Ocimum tenuiflorum

Ocimum tenuiflorum, holy basil or tulsi (तुलसी), synonym Ocimum sanctum, is a Lamiaceae perennial herb native to tropical Asia, sacred in Hinduism, rich in eugenol and ursolic acid, and studied in stress and metabolic clinical trials.

Oenothera

Oenothera (evening primroses) is a New World genus of about 140-156 herbaceous species in Onagraceae, famous as de Vries's mutation-theory plant, with biparental chloroplast inheritance, Renner complexes, and seed oil rich in gamma-linolenic acid.

Oenothera biennis

Oenothera biennis, the common evening primrose, is a biennial herb native to North America with bright yellow flowers that open at night, grown as the source of evening primrose oil.

Olacaceae

Olacaceae (R.Br. 1818, nom. cons.) is a pantropical family of Santalales with trees, shrubs, and climbers, about 25-29 genera and roughly 250 species; traditionally circumscribed Olacaceae is paraphyletic and has been split into narrower families including Erythropalaceae and Strombosiaceae.

Olea

Olea is a genus of about 32 evergreen Oleaceae trees and shrubs of the Old World, including the olive Olea europaea with six subspecies, over 1,000 cultivars, and the wild oleaster var. sylvestris.

Oleaceae

Oleaceae, the olive family (Hoffmanns. & Link, 1809), is a family of Lamiales with 25-27 genera and c. 600-615 species of trees, shrubs, and woody climbers, including Olea europaea, Fraxinus, Jasminum, and Syringa, recognized by opposite estipulate leaves and 4-merous flowers with 2 stamens.

Olearia

Olearia (daisy bush), genus of evergreen Asteraceae shrubs and small trees, about 180 species in Australia, New Zealand, and New Guinea; shown polyphyletic since 2002 and being split into Shawia and other genera.

Olive

Olive (Olea europaea), Mediterranean evergreen tree of Oleaceae cultivated for table fruit and olive oil; wild form is the oleaster. Levant domestication from Chalcolithic times, alternate bearing, oleuropein bitterness, Xylella threat.

Olive oil

Olive oil, the oil pressed from Olea europaea fruit, graded extra virgin (free acidity ≤0.8%), virgin, refined, or pomace by IOC and USDA standards; EU produces ~67% of world supply; UC Davis studies found many imported "extra virgin" oils fail standards.

Onagraceae

Onagraceae, the evening primrose family (Onagraceae Juss., nom. cons., 1789), a Myrtales family of about 650 species in 22-23 genera, mostly New World herbs and shrubs with 4-merous flowers, inferior ovaries, and viscin-threaded pollen; includes Oenothera, Fuchsia, Clarkia, Epilobium, Ludwigia.

Onobrychis

Onobrychis (sainfoins), Fabaceae genus published by Miller in 1754, native Europe to Mongolia, Arabia, Ethiopia; about 130–208 species; includes forage legume sainfoin O. viciifolia, non-bloating, tannin-rich.

Onobrychis viciifolia

Onobrychis viciifolia (common sainfoin) is a perennial Fabaceae forage legume native to Central and SE Europe, non-bloating due to condensed tannins, drought tolerant via a deep taproot, and under study as a protein-rich perennial pulse crop.

Ononis

Ononis (restharrow), Fabaceae genus of about 88 annual/perennial herbs and shrubs, Mediterranean-centered, monadelphous stamens, dimorphic anthers, used in medicine and as forage.

Operculina

Operculina Silva Manso is a pantropical genus of climbing vines in Convolvulaceae, type species O. turpethum, defined by an operculate capsule; includes medicinal jalapa and turpeth species.

Opuntia

Opuntia (prickly pear, cactus pear) is a genus of about 150 American cacti with jointed cladodes, glochids, and edible fruits, used for food, forage, and cochineal dye, and invasive in many regions.

Opuntia decumbens

Opuntia decumbens Salm-Dyck is a prickly pear cactus in the family Cactaceae, native from Mexico to Central America, first published in 1834 in the Hortus Dyckensis.

Opuntia leucotricha

Opuntia leucotricha, called nopal duraznillo in Spanish and Aaron's-beard prickly-pear in English, is a prickly pear cactus from the Altiplano of central Mexico that grows 3 to 4 m tall with white hair-like spines.

Orbea

Orbea Haw. (1812) is a genus of about 60 stem-succulent stapeliads in Apocynaceae, native to Africa and the Arabian Peninsula, with carrion-scented fly-pollinated flowers; type species Orbea variegata.

Oregano

Oregano, chiefly Origanum vulgare (wild marjoram), a Lamiaceae perennial herb of the Mediterranean; flavor from carvacrol and thymol; Mexican oregano is Lippia graveolens; world trade exceeds 15,000 tonnes.

Origanum

Origanum is a Lamiaceae genus of about 40-44 aromatic species native from Macaronesia and Europe to Central China, 90% Mediterranean. Includes oregano (O. vulgare, cymyl/carvacrol chemotype) and sweet marjoram (O. majorana, sabinyl chemotype).

Ornithopus

Ornithopus (bird's-foot, serradella) is a genus of six annual legume species in Fabaceae, tribe Loteae, native from Macaronesia and Europe to Iran and disjunctly to South America; O. sativus is a widely sown forage legume on acidic sandy soils.

Ornithopus sativus

Ornithopus sativus, known as French serradella or pink serradella, is an annual legume in the pea family, native to southwestern Europe and northwestern Africa and grown as forage.

Orobanchaceae

Orobanchaceae (broomrape family), Ventenat 1799, is the largest family of parasitic flowering plants in Lamiales, with over 2,000 species in about 90-115 genera, spanning non-parasitic, hemiparasitic, and holoparasitic members including Striga and Orobanche crop pests.

Orobanche

Orobanche (broomrapes), holoparasitic Orobanchaceae genus published 1753; chlorophyll-free root parasites on Asteraceae and other hosts; 174 species in 2025 broad circumscription; crop pests like O. crenata and O. cumana.

Orthosiphon

Orthosiphon (syn. Clerodendranthus) is a genus of 45 accepted Lamiaceae species native from Colombia and Africa to northeast Australia; its best-known species, O. aristatus or O. stamineus (cat's whiskers, misai kucing), is a Southeast Asian medicinal plant used as a diuretic and studied for antidiabetic effects.