Last updated October 1, 2009
| Name | Family | Nativity |
| Ochagavia carnea | Bromeliaceae | Chile |
| Ochna serrulata | Ochnaceae | South Africa |
| Odontonema strictum | Acanthaceae | Central America |
| Oenothera sp. | Onagraceae | |
| Oenothera speciosa | Onagraceae | SW United States, Mexico |
| Olea europaea | Oleaceae | Mediterranean |
| Olea europaea cv. Manzanillo | Oleaceae | |
| Olea exasperata | Oleaceae | South Africa |
| Olmediella betschleriana | Flacourtiaceae | Guatemala |
| Oncidium flexuosum | Orchidaceae | Brazil |
| Oncidium sp. | Orchidaceae | |
| Oncidium splendidum | Orchidaceae | Guatemala |
| Operculicarya decaryi | Anacardiaceae | Madagascar |
| Ophiopogon japonicus | Convallariaceae | Japan |
| Ophiopogon japonicus cv. Nana | Convallariaceae | |
| Opuntia auberi | Cactaceae | Mexico, Cuba |
| Opuntia basilaris | Cactaceae | SW United States |
| Opuntia basilaris v. treleasei | Cactaceae | SW United States |
| Opuntia caracassana | Cactaceae | Venezuela |
| Opuntia caribaea | Cactaceae | Dominican Republic |
| Opuntia decumbens | Cactaceae | S Mexico to Costa Rica |
| Opuntia dillenii | Cactaceae | SE US to Mex., Ecuador, Carib. |
| Opuntia echios | Cactaceae | Galapagos Islands |
| Opuntia echios v. gigantea | Cactaceae | Galapagos Islands |
| Opuntia elata | Cactaceae | Paraguay |
| Opuntia engelmannii v. cv. linguiformis | Cactaceae | Cen Texas |
| Opuntia engelmannii v. linguiformis | Cactaceae | |
| Opuntia galapageia | Cactaceae | Galapagos Islands |
| Opuntia galapageia v. galapageia | Cactaceae | Galapagos Islands |
| Opuntia gosseliniana | Cactaceae | Arizona to NW Mexico |
| Opuntia helleri | Cactaceae | Galapagos Islands |
| Opuntia lutea | Cactaceae | Mexico to Nicaragua |
| Opuntia megasperma v. megasperma | Cactaceae | Galapagos Islands |
| Opuntia megasperma v. mesophytica | Cactaceae | Galapagos Islands |
| Opuntia microdasys | Cactaceae | N Mexico |
| Opuntia microdasys v. monstrosa | Cactaceae | N Mexico |
| Opuntia microdasys v. rufida | Cactaceae | N Mexico |
| Opuntia quimilo | Cactaceae | N Argentina, Bolivia |
| Opuntia robusta | Cactaceae | Cen Mexico |
| Opuntia sp. | Cactaceae | |
| Opuntia sp. | Cactaceae | Galapagos Islands |
| Opuntia tomentosa | Cactaceae | Mexico, Guatemala |
| Oreocereus celsianus | Cactaceae | Bolivia, Peru, N Argentina |
| Oreocereus celsianus v. fossulatus | Cactaceae | Bolivia, Peru, N Argentina |
| Oreocereus doelzianus | Cactaceae | Cen Peru |
| Oreocereus hendriksenianus | Cactaceae | Chile, S Peru |
| Oreocereus hendriksenianus v. densilanatus | Cactaceae | Chile, S Peru |
| Oreocereus hybrid | Cactaceae | |
| Oreocereus leucotrichus | Cactaceae | Chile, S Peru |
| Oreocereus sp. | Cactaceae | |
| Oreocereus trollii | Cactaceae | N Argentina |
| Origanum dictamnus | Lamiaceae | Greece |
| Ornithogalum longibracteatum | Hyacinthaceae | South Africa |
| Ornithogalum viveus | Hyacinthaceae | |
| Orthophytum cv. Copper Penny | Bromeliaceae | |
| Orthophytum gurkenii | Bromeliaceae | E Brazil |
| Orthophytum lemei | Bromeliaceae | Brazil |
| Orthophytum vagans | Bromeliaceae | Brazil |
| Osmanthus fragrans | Oleaceae | E Asia |
| Osmanthus heterophylla cv. Argenteo-marginata | Oleaceae | |
| Osmunda regalis cv. Purpurescens | Osmundaceae | |
| Otatea acuminata ssp. aztecorum | Poaceae | Mexico to Nicaragua |
| Otatea glauca | Poaceae | Mexico(Chiapas) |
| Otholobium fruticans | Fabaceae | South Africa |
| Othonna protecta | Asteraceae | South Africa, Namibia |
| Oxalis carnosa | Oxalidaceae | Bolivia, Chile, Peru,Galapagos |
| Oxalis gigantea | Oxalidaceae | Chile |
