Common names: O
- oak
- oak fern
- oak-leaf lettuce
- oak-leaved hydrangea
- oakleaf hydrangea
- oat
- obedient plant
- ocean blue morning glory
- ocean spray
- Ohio buckeye
- old man of the Andes
- old man's beard
- Oldham's blueberry
- oleander
- olive tree
- Oliver's touch-me-not
- Omei rose
- one-leaf onion
- oneflower clover
- onion
- opium poppy
- orache
- orange
- orange ball buddleja
- orange blossom
- orange cestrum
- orange cone flower
- orange hawkweed
- orange lily
- orange mullein
- orange trumpet vine
- orchard apple
- orchid dahlia
- oregano
- Oregon grape
- oriental arborvitae
- oriental beech
- oriental bittersweet
- oriental campion
- oriental cherry
- oriental hornbeam
- oriental knotweed
- oriental lily
- oriental paperbush
- oriental photinia
- oriental plane
- oriental poppy
- oriental spruce
- oriental trumpet lily
- ornamental allium
- ornamental bacopa
- ornamental onion
- ornamental strawberry
- orpine
- Osage orange
- Oshima cherry
- ostrich fern
- Oudtshoorn pincushion
- outstanding dendrobium
- ox-eye daisy
- oxeye daisy
- Ozark witch hazel