Plants — O
- Ochna integerrima Yellow Mai
- Ochradenus baccatus Taily Weed
- Ocimum Basilic
- Ocimum americanum American Basil
- Ocimum basilicum basilic
- Ocimum basilicum var. minimum
- Ocimum campechianum
- Ocimum gratissimum Basilic africain, Basilic sauvage, Baumier, Gros baume, Menthe gabonaise
- Ocimum menthifolium
- Ocimum tenuiflorum Basilic sacré
- Oenocarpus bataua
- Oenothera Œnothère
- Oenothera biennis Onagre bisannuelle
- Oenothera pubescens South American evening primrose
- Oenothera triloba Onagre acaule
- Okoubaka aubrevillei Death Tree
- Olax psittacorum Bois De Perroquet
- Olea europaea olivier européen
- Olea europaea subsp. cuspidata Olea europaea cuspidata
- Ononis spinosa Bugrane épineuse
- Operculina turpethum
- Ophiocordyceps sinensis Caterpillar Fungus
- Ophioglossum vulgatum Adder's Tongue
- Ophiopogon japonicus muguet du Japon
- Oplopanax Devil's Clubs and Walking Sticks
- Opuntia Cactus raquettes
- Opuntia ficus-indica figuier de Barbarie
- Opuntia matudae
- Orchis Orchids [genus]
- Orchis mascula Orchis mâle
- Orchis militaris Orchis guerrier
- Origanum majorana Marjolaine
- Origanum onites Origan
- Origanum syriacum Bible Hyssop
- Origanum vulgare Marjolaine
- Ornithogalum Ornithogale
- Ornithogalum umbellatum Ornithogale en ombelle
- Orostachys japonica Rock pine
- Oroxylum indicum midnight horror
- Orthilia secunda Pirole unilatérale
- Orthosiphon aristatus Moustaches de chat
- Orthosiphon stamineus
- Osbeckia
- Osmanthus fragrans Olivier odorant
- Oxalis acetosella Common Wood Sorrel
- Oxalis corniculata
- Oxalis obtriangulata Large wood sorrel
- Oxalis stricta
- Oxybasis rubra Ansérine rouge
- Oxydendrum arboreum Oxydendrum
- Oxystelma esculentum
- Oxytropis ochrocephala