| NAME OF PLANT | WHERE FOUND | DATE | WHERE FOUND | DATE |
| Oenothera biennis | ||||
| common evening-primrose | ||||
| Oenothera glazioviana | ||||
| large-flowered evening-primrose | ||||
| Onobrychis viciifolia | ||||
| sainfoin | ||||
| Ononis repens | ||||
| common restharrow | ||||
| Ononis spinosa | ||||
| spiny rest-harrow | ||||
| Onopordum acanthium | ||||
| cotton thistle | ||||
| Ophioglossum vulgatum | ||||
| adder's-tongue | ||||
| Ophrys apifera | ||||
| bee orchid | ||||
| Ophrys insectifera | ||||
| fly orchid | ||||
| Orchis mascula | ||||
| early purple orchid | ||||
| Orchis morio | ||||
| green-winged orchid | ||||
| Oreopteris limbosperma | ||||
| lemon scented fern | ||||
| Origanum vulgare | ||||
| wild marjoram | ||||
| Ornithogalum angustifolium | ||||
| star-of-Bethlehem | ||||
| Ornithopus perpusillus | ||||
| bird's-foot | ||||
| Orobanche elatior | ||||
| knapweed broomrape | ||||
| Orobanche hederae | ||||
| ivy broomrape | ||||
| Orobanche minor | ||||
| common broomrape | ||||
| Osmunda regalis | ||||
| royal fern | ||||
| Oxalis acetosella | ||||
| wood-sorrel | ||||
| 61 | ||||