Northern catalpa 
Nothochelone 
Obedient Plant 
Obscure indian paintbrush 
Odontites aucheri 
Odontites linkii 
Olive Tree 
One-Flowered Cancer Root 
Orange trumpet 
Oregano 
Oreocharis longituba 
Oriental Bugle 
Origanum dayi 
Origanum ramonense 
Origanum rotundifolium 
Origanum syriacum 
Orobanche anatolica 
Orobanche camptolepis 
Orobanche cernua 
Orobanche daninii 
Orobanche fasciculata 
Orobanche hermonis 
Orobanche schultzii 
Otostegia fruticosa