Index of plants by genus
| Species | Common Name | Family |
| Oclemena acuminata | Whorled Aster | Asteraceae |
| Oclemena nemoralis | Bog Aster | Asteraceae |
| Oenothera biennis | Common Evening Primrose | Onagraceae |
| Oenothera clelandii | Sand Evening Primrose | Onagraceae |
| Oenothera laciniata | Cutleaf Evening Primrose | Onagraceae |
| Oenothera parviflora | Northern Evening Primrose | Onagraceae |
| Oenothera perennis | Small Sundrops | Onagraceae |
| Oenothera pilosella | Prairie Sundrops | Onagraceae |
| Onoclea sensibilis | Sensitive Fern | Dryopteridaceae |
| Ophioglossum pusillum | Northern Adder's Tongue | Ophioglossaceae |
| Opuntia fragilis | Brittle Prickly Pear | Cactaceae |
| Opuntia humifusa | Eastern Prickly Pear | Cactaceae |
| Opuntia macrorhiza | Plains Prickly Pear | Cactaceae |
| Orobanche uniflora | One-flowered Cancer Root | Orobanchaceae |
| Orontium aquaticum | Golden Club | Araceae |
| Orthilia secunda | One-sided Wintergreen | Ericaceae |
| Oryzopsis racemosa | Black-seeded Rice Grass | Poaceae |
| Osmorhiza claytonii | Hairy Sweet Cicely | Apiaceae |
| Osmorhiza longistylis | Sweet Cicely | Apiaceae |
| Osmunda cinnamomea | Cinnamon Fern | Osmundaceae |
| Osmunda claytonia | Interrupted Fern | Osmundaceae |
| Osmunda regalis | Royal Fern | Osmundaceae |
| Ostrya virginiana | Hop-hornbeam | Betulaceae |
| Oxalis montana | Mountain Woodsorrel | Oxalidaceae |
| Oxalis stricta | Yellow Woodsorrel | Oxalidaceae |
| Oxalis violacea | Violet Woodsorrel | Oxalidaceae |
| Oxydendrum arboreum | Sourwood | Ericaceae |
© 2004 Michael Hough