Oak
Oakes' Evening Primrose
Oakes' Pondweed
Oakfern
Oakforest Woodrush
Oakleaf Goosefoot
Oat
Oatgrass
Obedient Plant
Oconee Bells
Ogden's Pondweed
Ohio Polytrichum Moss
Okra
Old-man-in-the-spring
Oldfield Blackberry
Oldpasture Bluegrass
Oldwoman
Oleaster
One-cone Clubmoss
Oneflowered Broomrape
Onescale Spikerush
Oneseed Bur Cucumber
Oneseed Hawthorn
Onion
Ontario Lobelia
Openfield Sedge
Opium Poppy
Orange Coneflower
Orange Daylily
Orange Eye Butterflybush
Orange Hawkweed
Orange Lichen
Orange Mullein
Orangefruit Horse-gentian
Orchardgrass
Oregano
Oriental Bittersweet
Oriental Lady's Thumb
Oriental Photinia
Origanum
Ornamental Jewelweed
Orthilia
Osage Orange
Osmunda