| O |
| Ocimum basilicum, Sweet basil |
| Odontonema strictum, Firespike |
| Oenothera biennis, Evening primrose |
| Oenothera fruticosa, Sundrops |
| Oenothera speciosa, Showy evening primrose |
| Oenothera tetragona, Sundrops |
| Omphalodes linifolia, Navelwort |
| Onoclea sensibilis, Sensitive fern; Bead fern |
| Ophiopogon jaburan vittata, Snakebeard; White lilyturf |
| Ophiopogon japonicus 'Nana', Dwarf mondo grass |
| Ophiopogon japonicus, Mondo grass |
| Ophiopogon planiscapus 'Arabicus', Black mondo grass |
| Origanum majorana, Sweet marjoram |
| Origanum vulgare, Oregano |
| Ornithogalum, Star of Bethlehem |
| Orontium aquaticum, Golden club |
| Orontium aquaticum, Golden club |
| Osmanthus americanus, Devilwood; Wild olive |
| Osmanthus americanus, Wild olive; Devilwood |
| Osmanthus delavayi, Delavay teaolive |
| Osmanthus fragrans, Fragrant tea olive |
| Osmanthus heterophyllus, Holly tea olive; Holly osmanthus; False holly |
| Osmanthus x burkwoodii, Burkwood osmanthus |
| Osmanthus x fortunei, Fortune's osmanthus |
| Osmunda cinnamomea, Cinnamon fern |
| Osmunda claytoniana, Interrupted fern; Clayton's fern |
| Osmunda regallis, Royal fern; Flowering fern |
| Osteospermum hyoseroides, Cape daisy; South African daisy |
| Ostrya virginiana, Hop hornbeam |
| Oxalis (Tender species), Wood-sorrel; Oxalis |
| Oxalis crassipes, Oxalis |
| Oxalis grandis, Sourgrass; Oxalis; Large yellow wood-sorrel |
| Oxalis montana, Common wood sorrel; Sourgrass |
| Oxalis spp., Wood sorrel; Wood shamrock |
| Oxalis violacea, Violet wood sorrel |
| Oxydendrum arboreum, Sourwood; Lily-of-the-valley tree |
| Oxypetalum caeruleum = Tweedia caerulea, Tweedia |