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▶ Oenanthe
Common name: Water dropwort
▶ Oenothera
Common name: Evening-primrose
▶ Oldenlandia
Common name: Mille-graines
▶ Onobrychis
Common name: Sainfoin
▶ Onopordum
Common name: Cotton-thistle
▶ Oplismenus
Common name: Basketgrass, mountaingrass
▶ Opuntia
Common name: Prickly-pear, cholla
▶ Oreocarya
Common name: Perennial cat's-eye
▶ Origanum
Common name: Marjoram
▶ Ornithogalum
Common name: Star-of-Bethlehem, false sea-onion
▶ Orobanche
Common name: Broomrape
▶ Orontium
Common name: Goldenclub
▶ Orthocarpus
Common-name: Owl-clover
▶ Oryzopsis
Common name: Mountain ricegrass
▶ Osmorhiza
Common name: Sweet-cicely
▶ Osteospermum
Common name: daisy-bush
▶ Oxalis
Common name: Wood-sorrel