Plants
- Oak leaved hydrangea 'Snowflake'Hydrangea quercifolia 'Snowflake'
- Oakleaf hydrangeaHydrangea quercifolia
- Obedient Plant 'Summersnow'Physostegia virginiana 'Summersnow'
- Obedient-plant Physostegia virginiana
- Ocimum basilicum 'African Blue' Ocimum basilicum 'African Blue'
- Ocinum basilicum 'Genovese Gigante'Ocinum basilicum 'Genovese Gigante'
- Odontioda orchid 'Le Hocq'Odontioda 'Le Hocq'
- Odontioda orchid 'Lilly Menuet'Odontioda 'Lilly Menuet'
- Odontocidium orchid 'Big Mac Saragosa'Odontocidium 'Big Mac Saragosa'
- Odontocidium orchid 'Hansueli isler'Odontocidium 'Hansueli isler'
- Odontocidium orchid 'Tiger Crow' - 'Golden Girl'Odontocidium 'Tiger Crow' - 'Golden Girl'
- Odontocidium orchid 'Tiger Hambuhren'Odontocidium 'Tiger Hambuhren'
- Odontoglossum orchidOdontoglossum cirrhosum
- Odontonia orchid 'Oxbow'Odontonia Debutante 'Oxbow'
- Oenothera fruticosa 'Fyrverkeri'Oenothera fruticosa 'Fyrverkeri'
- Oenothera MacrocarpaOenothera Macrocarpa
- Oenothera Speciosa 'Siskiyou'Oenothera Speciosa 'Siskiyou'
- okra
- Old Man of Mexico CactusCephalocereus senilis
- OleanderNerium oleander
- Oleaster 'Quicksilver'Elaeagnus angustifolia 'Quicksilver'
- Olive treeOlea europaea
- Olws eyes CactusMammillaria parkinsonii
- Olympian MulleinVerbascum olympicum
- Omphalodes Cappadocica 'Starry Eyes'Omphalodes Cappadocica 'Starry Eyes'
- Oncidium orchidOncidium ornithorrhyncum
- Oncidium Tsuiku MargueriteOncidium Tsuiku Marguerite
- OnionAllium cepa
- OnionAllium carinatum ssp. pulchellum
- Onion Red Barron (GS)
- Onion Red Creole
- Onions 'Carmen'Allium cepa 'Carmen'
- Ontario Poplar 'Aurora'Populus candicans 'Aurora'
- OPHIOPOGON jaburanOPHIOPOGON jaburan
- Ophiopogon planiscapus 'Niger'Ophiopogon planiscapus 'Niger'