| Oak-leaf Fern | Drynaria quercifolia |
| Oat | Avena sativa |
| Oblong-leafed Aglaonema | Aglaonema nitidum |
| Obscure Morning Glory | Ipomoea obscura |
| Octopus Tree | Heptapleurum actinophyllum |
| Ogiera | Eleutheranthera ruderalis |
| Oil Palm | Elaeis guineensis |
| Okra | Abelmoschus esculentus |
| Okinawa Spinach | Gynura bicolor |
| Old Man's Beard | Clematis aristata |
| Oleander | Nerium oleander |
| Old World Climbing Fern | Lygodium microphyllum |
| One-leaf Clover | Alysicarpus vaginalis |
| Onion | Allium cepa |
| Orange Cosmos | Cosmos sulphureus |
| Orange Jasmine | Murraya paniculata |
| Orange Shrimp Plant | Aphelandra sinclairiana |
| Orange Tulip Ginger | Costus productus |
| Orchid Tree | Bauhinia purpurea |
| Ordeal Tree | Erythrophleum suaveolens |
| Oriental Arborvitae | Platycladus orientalis |
| Oriental False Hawksbeard | Youngia japonica |
| Oriental Thuja | Platycladus orientalis |
| Ornamental Banana | Musa ornata |
| Oval Leaf False Buttonweed | Spermacoce latifolia |
| Oval-leaved Mistletoe | Viscum ovalifolium |
| Oxblood Ginger | Costus erythrophyllus |
| Oyster Plant | Tradescantia spathacea |