| O |
| Oak, Blackjack, Quercus merilandica |
| Oak, Bluejack, Quercus incana |
| Oak, Bur, Quercus macrocarpa |
| Oak, Chestnut, Quercus prinus |
| Oak, Chinkapin, Quercus muchlenbergii |
| Oak, Columnar English, Quercus robur 'Fastigiata' |
| Oak, Daimyo, Quercus dentala |
| Oak, Durmast, Quercus petraea |
| Oak, English, Quercus robur |
| Oak, Georgia, Quercus georgiana |
| Oak, Henry tanbark, Lithocarpus henryi |
| Oak, Japanese blue, Quercus glauca |
| Oak, Laurel, Quercus hemisphaerica |
| Oak, Live, Quercus virginiana |
| Oak, Michaux, Quercus michauxii |
| Oak, Oriental, Quercus variabilis |
| Oak, Overcup, Quercus lyrata |
| Oak, Pin, Quercus palustris |
| Oak, Post, Quercus stellata |
| Oak, Red, Quercus rubra |
| Oak, Ring cupped, Quercus glauca |
| Oak, Sand live, Quercus geminata |
| Oak, Sand post, Quercus margarettiae |
| Oak, Sawtooth, Quercus acutissima |
| Oak, Scarlet, Quercus coccinea |
| Oak, Sessile, Quercus petraea |
| Oak, Shingle, Quercus imbricaria |
| Oak, Shumard, Quercus shumardii |
| Oak, Southern red, Quercus falcata |
| Oak, Swamp chestnut, Quercus michauxii |
| Oak, Swamp laurel, Quercus lurifolia |
| Oak, Swamp white, Quercus bicolor |
| Oak, Truffle, Quercus robur |
| Oak, Turkey, Quercus laevis |
| Oak, Water, Quercus nigra |
| Oak, White, Quercus alba |
| Oak, Willow, Quercus phellos |
| Oak, Willow, Quercus phellos |
| Oats, Wild; Sessile-leaf bellwort, Uvularia sessilifolia |
| Obedient plant; False dragonhead, Physostegia virginianum |
| Oconee bells; Shortia, Shortia galacifolia |
| Octoberfest rose, Rosa 'MAClanter' |
| Odorata water lily, Nymphaea 'Odorata' |
| Old Blush China rose, Rosa 'Old Blush China' |
| Old Blush rose, Rosa 'Old Blush' |
| Old Crimson China rose, Rosa 'Old Crimson China' |
| Old Gray Hill Red China rose, Rosa 'Old Gray Hill' |
| Old Pink Dailey rose, Rosa 'Old Pink Dailey' |
| Oldman's beard, Chionanthus virginicus |
| Ole rose, Rosa 'Ole' |
| Oleander, Nerium oleander |
| Olive, Russian, Eleagnus angustifolia |
| Olive, Wild, Osmanthus americanus |
| Olive, Wild, Osmanthus americanus |
| Olympiad rose, Rosa 'MACauck' |
| Onion, Garlic, Allium moly (A. luteum) |
| Onion, Ornamental, Allium aflatunense |
| Onion, Ornamental, Allium christophii (A. albopilosum) |
| Onion, Ornamental, Allium giganteum |
| Onion, Ornamental, Allium karataviense |
| Onion, Ornamental, Allium oreophilum (A. ostrowskianum) |
| Onion, Ornamental, Allium sphaerocephalon |
| Onion, Ornamental, Allium caeruleum (A. azureum) |
| Onion, Ornamental, Allium cowanii |
| Onion, Ornamental, Allium neapolitanum |
| Onions, Ornamental, Allium |
| Opalia rose, Rosa 'NOAschnee' |
| Opening Night rose, Rosa 'JAColber' |
| Ophelia rose, Rosa 'Ophelia' |
| Orange clockvine, Thunbergia gregorii |
| Orange Honey rose, Rosa 'Orange Honey' |
| Orange Slippers rose, Rosa 'Orange Slippers' |
| Orange Triumph rose, Rosa 'Orange Triumph' |
| Orange, Hardy; Trifoliate orange, Poncirus trifoliata |
| Orange, Mexican, Choisya ternata |
| Orchid, Chinese ground, Bletilla striata |
| Orchid, Chinese ground, Bletilla striata |
| Orchid, Cranefly, Tipularia discolor |
| Orchid, Green fringed; Ragged fringed orchid, Habenaria lacera |
| Orchid, Hardy, Bletilla striata |
| Orchid, Large round-leaf; Round leaved orchis, Habenaria orbiculata = Platanthera orbiculata var. orbiculata |
| Orchid, Purple fringed, Platanthera (Habenaria) psycodes |
| Orchid, Rose crested; Rose pogonia, Pogonia ophioglossoides |
| Orchid, Yellow fringed, Habenaria ciliaris |
| Orchid, Yellow fringed, Platanthera ciliaris = Habenaria ciliaris |
| Orchids, Fringed, Habenaria spp. |
| Orchis, Long-bracted, Habenaria viridis var. bracteata |
| Orchis, Pale green; Southern rain-orchis, Habenaria flava |
| Orchis, Showy, Galearis spectablis (Orchis spectabillis) |
| Oregano, Origanum vulgare |
| Oregold rose, Rosa 'TANolg' |
| Oriental fountain grass, Pennisetum orientale |
| Osageorange; Hedgeapple, Maclura pomifera |
| Osmanthus, Burkwood, Osmanthus x burkwoodii |
| Osmanthus, Fortune's, Osmanthus x fortunei |
| Osmanthus, Holly, Osmanthus heterophyllus |
| Ostrich fern, Matteuccia struthiopteris; M. pennsylvanica; Pteris nodulosa |
| Outta the Blue rose, Rosa 'WEKstephitsu' |
| Ox-eye, Heliopsis helianthoides |
| Oxalis, Oxalis (Tender species) |
| Oxalis, Oxalis crassipes |
| Oxalis; Sourgrass; Sorrel, Large yellow wood, Oxalis grandis |
| Oyster-plant, Tragopogon porrifolius |