- Obedient plant
- Obscure Morning Glory
- Ocean spray
- Oceanblue morning glory
- Ocotillo
- Ohio spiderwort
- Old man's beard
- Old Man's Beard Clematis
- Old World Diamond Flower
- Oleander
- Olive
- One-flowered Wintergreen
- One-sided wintergreen
- Oneseed Bur Cucumber
- Onion-Leafed Asphodel
- Opium poppy
- Opposite-leaved Golden-saxifrage
- Orange bush monkeyflower
- Orange day-lily
- Orange hawkweed
- Orange honeysuckle
- Orange Jasmine
- Orange milkwort
- Orangefruit Passionflower
- Orchard grass
- Orcutt's Woodyaster
- Oregano
- Oregon Beaked Moss
- Oregon Boxwood
- Oregon grape
- Oregon oak
- Oriental false hawksbeard
- Orpine
- Osage-orange
- Osoberry
- Ostrich fern
- Ostrich-plume Moss
- Oxeye daisy
- Oxlip