'Oakleaf' Heartleaf Foamflower Oakleaf Hydrangea (spp.)
Oakleaf Hydrangea 'Alice' Oakleaf Hydrangea 'Allison'
Oakleaf Hydrangea 'Brenhill (Gatsby Gal™)' Oakleaf Hydrangea 'Gatsby Moon™'
Oakleaf Hydrangea 'Gatsby Star™' Oakleaf Hydrangea 'Harmony'
Oakleaf Hydrangea 'Semmes Beauty' Oakleaf Hydrangea 'Snow Queen'
Oakleaf Hydrangea 'Snowflake' Oakleaf Hydrangea 'Syke's Dwarf'
Oats, North American Wild Oats, Northern Sea
Oats, River Oats, Sea
Oats, Spanglegrass River Oats, Wild
Obedient Plant (spp.) Obedient Plant 'Miss Manners'
Obedient Plant 'Mordea Beauty' Obedient Plant 'Pink Bouquet'
Obedient Plant 'Red Beauty' Obedient Plant 'Summer Snow'
Obedient Plant Variegated Oblong Fern
Ocean Spray Oconee Bells
Ocotillo Ocotillo Macho
Ocotillo, Tree Ocotillo: macdougalii
Ocotillo: purpusii Ocotillo: splendens
Octopus Agave Octopus Plant: actinophylla (spp.)
Octopus Plant: actinophylla 'Amate™' Octopus Tree (spp.)
Octopus Tree 'Amate™' Ogeechee Lime
Ogeechee Tupelo Ohio Buckeye (spp.)
Ohio Buckeye 'Monticola' Ohio Buckeye 'Sargentii'
Ohio Buckeye: glabra pallida Ohio Spiderwort 'Mrs. Loewer'
'Oklahoma' Oklahoma Redbud Oklahoma Redbud 'Oklahoma'
Oklahoma Redbud 'Texas White' 'Old Gold' American Holly
Old Man Cactus Old Man Palm
Old Man's Beard Old Man's Beard: virginicus
Old Man's Whiskers Old Woman (spp.)
Old Woman 'Silver Brocade' Olive, Texas Wild
Olive, Wild Olive: americanus, Wild
'Olympic Fire' Mountain Laurel 'Omega Skyrocket' Stokes' Aster
One Day Lily Onion, Flowering (spp.)
Onion, Flowering 'Pink Feathers' Onion, Flowering 'Schnittlauch'
Onion, Nodding Onion, Pacific
Onion, Prairie Onion, Swamp
Onion, Tapertip Onion, Wild
Orange Coneflower 'Goldsturm' 'Orange Jacket' French Marigold
Orange Larkspur (spp.) Orange Larkspur 'Laurin'
Orange Sneezeweed Orange Sunflower (spp.)
Orange Sunflower 'Ballerina' Orange Sunflower 'Summer Sun'
Orange, Osage (spp.) Orange, Osage 'Double O'
Orchid Tree: ramosissima Orchid, Fringed
Orchid, Ladyslipper Orchid, Ragged
Orchid: radiata Orcutt Aster