Orange agoseris 
Orange Coneflower 
Orange Hawkweed 
Orange Immortelle 
Oregon Gumplant 
Oriental false hawksbeard 
Ormenis mixta 
Osteospermum vaillantii 
Ox-eye daisy 
Oxeye 
Oxford Ragwort 
Pachystegia insignis 
Packera cana 
Packera pseudaurea 
Pale agoseris 
Pale Bellflower 
Pale Indian plantain 
Pale Purple Coneflower 
Pale-leaved Woodland Sunflower 
Palmate Umbrella Plant 
Palmleaf Thoroughwort 
Panakenake 
Panicled Aster 
Partridge-Feather