O
- Obolaria virginica (N)
- Oclemena acuminata (N)
- Oenanthe aquatica (I)
- Oenothera biennis (N)
- Oenothera clelandii (N)
- Oenothera fruticosa ssp. fruticosa (N)
- Oenothera fruticosa ssp. glauca (N)
- Oenothera laciniata (N)
- Oenothera nutans (N)
- Oenothera oakesiana (N)
- Oenothera parviflora (N)
- Oenothera perennis (N)
- Oenothera pilosella ssp. pilosella (N)
- Oenothera speciosa (N)
- Oenothera triloba (N)
- Oenothera villosa ssp. villosa (N)
- Oligoneuron album (N)
- Oligoneuron ohioense (N)
- Oligoneuron riddellii (N)
- Oligoneuron rigidum var. glabratum (N)
- Oligoneuron rigidum var. rigidum (N)
- Onoclea sensibilis (N)
- Onopordum acanthium (I)
- Onosmodium bejariense var. hispidissimum (N)
- Ophioglossum engelmannii (N)
- Ophioglossum pusillum (N)
- Ophioglossum vulgatum (N)
- Opuntia humifusa (N)
- Opuntia macrorhiza var. macrorhiza (N)
- Orbexilum onobrychis (N)
- Orbexilum pedunculatum var. pedunculatum (N)
- Origanum vulgare (I)
- Ornithogalum nutans (I)
- Ornithogalum umbellatum (I)
- Orobanche ludoviciana ssp. ludoviciana (N)
- Orobanche uniflora (N)
- Orthilia secunda (N)
- Oryzopsis asperifolia (N)
- Osmorhiza claytonii (N)
- Osmorhiza longistylis (N)
- Osmunda cinnamomea var. cinnamomea (N)
- Osmunda claytoniana (N)
- Osmunda regalis var. spectabilis (N)
- Ostrya virginiana var. virginiana (N)
- Oxalis corniculata (N)
- Oxalis dillenii (N)
- Oxalis grandis (N)
- Oxalis montana (N)
- Oxalis stricta (N)
- Oxalis violacea (N)
- Oxydendrum arboreum (N)
- Oxypolis rigidior (N)
Read more about this topic: Flora Of Ohio