Oak          

Oak Mistletoe          

Oakleaf Fleabane          

Oat          

Oatgrass          

Obedient Plant          

Obolaria          

Obovate Beakgrain          

Oceanblue Phacelia          

Ohio Buckeye      

Oklahoma Grasspink          

Oklahoma Phlox          

Oklahoma Plum          

Oklahoma Sedge          

Old-man-in-the-spring      

Oldenlandia          

Oldfield Blackberry      

Oldfield Milkvine          

Oldfield Sneezeweed          

Oldplainsman          

Oleaster          

Oneflower Bedstraw          

Oneflower False Fiddleleaf          

Oneflower Flatsedge          

Oneflowered Broomrape          

Onerow Yellowcress          

Oneseed Bur Cucumber          

Oneseed Hawthorn      

Onion          

Ontario Blazing Star          

Openflower Rosette Grass          

Openground Draba          

Ophiopogon          

Oppositeleaf Spotflower          

Orange Coneflower          

Orange Daylily          

Orange Hawkweed          

Orangefruit Horse-gentian          

Orangegrass      

Orchardgrass          

Oregano          

Oriental Bittersweet      

Oriental Evening Primrose          

Oriental False Hawksbeard