| Ocimum basilicum |
Basil |
 |
| Oenothera (many) |
Evening Primrose |
 |
| Oenothera acaulis aurea |
Evening Primrose |
|
| Oenothera biennis |
Evening Primrose |
|
| Oenothera caespitosa |
Evening Primrose |
|
| Oenothera erythrosepala |
Evening Primrose |
|
| Oenothera missouriensis |
Missouri Evening Primrose |
|
| Olnea tesota |
Desert Ironwood |
 |
| Onobrychis viciifolia |
Sainfoin |
|
| Ononis (many) |
Rest Harrow |
|
| Ononis cenisia |
Rest Harrow |
|
| Ononis repens |
Rest Harrow |
|
| Ononis spinosa |
Spiny Rest Harrow |
|
| Onosma taurica |
Golden Drop, Golden-Flowered Onosma |
|
| Opuntia ficus-indica |
Indian Fig Opuntia |
|
| Origanum (many) |
Marjoram |
|
| Origanum vulgare |
Wild Marjoram |
|
| Orlaya grandiflora |
Whiter Lace Flower |
|
| Ornithogalum (many) |
Star of Bethlehem |
|
| Ornithogalum pyramidale |
Star of Bethlehem |
 |
| Orthrosanthus laxus |
Morning Iris |
|
| Osteospermum |
Cape Daisy |
 |
| Osteospermum barberae |
Trailing African Daisy |
|
| Ostrya carpinifolia |
Hop Hornbeam |
|
| Otholobium glandulosum |
Culen |
|
| Oxalis (many) |
Sorrel |
|
| Oxalis corniculatus |
Yellow Sorrel, Sleeping Beauty |
|
| Oxalis lactea (magellanica) |
Sorrel |
|
| Oxypetalum caeruleum |
Sorrel |
|
| Oxytropis (many) |
Milk Vetch |
|
| Oxytropis |
Milk Vetch |
|
