Plants — O
- Ochna integerrima Yellow Mai
- Ochradenus baccatus Taily Weed
- Ocimum Basil
- Ocimum americanum American Basil
- Ocimum basilicum basil
- Ocimum basilicum var. minimum
- Ocimum campechianum
- Ocimum gratissimum African basil
- Ocimum menthifolium
- Ocimum tenuiflorum Holy Basil
- Oenocarpus bataua
- Oenothera Evening Primrose
- Oenothera biennis Common Evening Primrose
- Oenothera pubescens South American evening primrose
- Oenothera triloba sessile evening-primrose
- Okoubaka aubrevillei Death Tree
- Olax psittacorum
- Olea europaea Olive
- Olea europaea subsp. cuspidata African olive
- Ononis spinosa
- Operculina turpethum
- Ophiocordyceps sinensis Caterpillar Fungus
- Ophioglossum vulgatum Adder's Tongue
- Ophiopogon japonicus Dwarf Lily-turf
- Oplopanax Devil's Clubs and Walking Sticks
- Opuntia Prickly Pears
- Opuntia ficus-indica Barbary fig
- Opuntia matudae
- Orchis Orchids [genus]
- Orchis mascula Early-Purple Orchid
- Orchis militaris Military Orchid
- Origanum majorana marjoram
- Origanum onites Pot Marjoram
- Origanum syriacum Bible Hyssop
- Origanum vulgare
- Ornithogalum
- Ornithogalum umbellatum
- Orostachys japonica Rock pine
- Oroxylum indicum midnight horror
- Orthilia secunda
- Orthosiphon aristatus Cat's Whiskers
- Orthosiphon stamineus
- Osbeckia
- Osmanthus fragrans Fragrant olive, Sweet olive, Tea olive
- Oxalis acetosella Common Wood Sorrel
- Oxalis corniculata
- Oxalis obtriangulata Large wood sorrel
- Oxalis stricta
- Oxybasis rubra Alkali-blite
- Oxydendrum arboreum Oxydendrum
- Oxystelma esculentum
- Oxytropis ochrocephala