Oak          

Oak Mistletoe          

Oakleaf Goosefoot          

Oat          

Oatgrass          

Obedient Plant          

Obovate Beakgrain          

Ohio Buckeye      

Oklahoma Beardtongue          

Oklahoma Blackberry          

Oklahoma Evening Primrose          

Oklahoma Grasspink          

Oklahoma Monkeyflower          

Oklahoma Phlox          

Oklahoma Plum          

Oklahoma Saxifrage          

Oklahoma Sedge          

Oklahoma Thelypody          

Old-man-in-the-spring      

Oldenlandia          

Oldfield Milkvine          

Oleaster          

Oneflower False Fiddleleaf          

Oneflower Flatsedge          

Oneflowered Broomrape          

Oneseed Bur Cucumber          

Oneseed Juniper      

Onion          

Openflower Rosette Grass          

Openground Draba          

Opium Poppy          

Oppositeleaf Bahia          

Oppositeleaf Spotflower          

Orange Coneflower          

Orangefruit Horse-gentian          

Orangegrass      

Orchardgrass          

Oregon Cliff Fern          

Osage False Foxglove          

Osage Orange      

Osmunda          

Othake          

Ouachita Bluet          

Ouachita False Indigo