Oak          

Oak Mistletoe          

Oakleaf Fleabane          

Oakleaf Hydrangea      

Oat          

Oatgrass          

Obedient Plant          

Obolaria          

Obovate Beakgrain          

Oceanblue Phacelia          

Oconee Bells          

Ohio Buckeye      

Oklahoma Grasspink          

Oklahoma Sedge          

Old-man-in-the-spring      

Oldenlandia          

Oldfield Blackberry      

Oldfield Milkvine          

Oldmaid Hawthorn          

Oldpasture Bluegrass          

Oleaster          

Oneflower False Fiddleleaf          

Oneflowered Broomrape          

Onerow Yellowcress          

Oneseed Bur Cucumber          

Oneseed Hawthorn      

Onion          

Ontario Blazing Star          

Openflower Rosette Grass          

Opium Poppy          

Oppositeleaf Spotflower          

Orange Coneflower          

Orange Daylily          

Orange Eye Butterflybush          

Orange Mullein          

Orangefruit Horse-gentian          

Orangegrass      

Orchardgrass          

Oriental Bittersweet      

Oriental False Hawksbeard          

Osage Orange      

Osmunda          

Ouachita Mountain Sedge          

Oval-leaf Knotweed