Oak          

Oakfern          

Oakleaf Mountain Ash          

Oat          

Oatgrass          

Obedient Plant          

Oblongleaf Basindaisy          

Oblongleaf Bluebells          

Obovate Buckthorn          

Obtuse Sedge          

Oceanspray      

Oil Shale Columbine          

Oil Shale Fescue          

Oilshale Cryptantha          

Old-man-in-the-spring      

Oleander      

Oleaster          

Oneflower Helianthella      

Oneflowered Broomrape          

Oneseed Hawthorn      

Oneseed Juniper      

Onespike Danthonia      

Onestem Fleabane          

Onion          

Oniongrass      

Onyxflower          

Opal Phlox          

Opium Poppy          

Orange Agoseris      

Orange Daylily          

Orange Honeysuckle          

Orange Lily          

Orchardgrass          

Oregon Boxleaf          

Oregon Buttercup          

Oregon Checkerbloom      

Oregon Cliff Fern          

Oregon Silene          

Oregon Willowherb          

Oreoxis          

Oriental Poppy          

Oriental Virginsbower          

Orpine Stonecrop          

Orthilia