| Oleaceae | Olive family |
| Onagraceae | Evening Primrose family |
| Onocleaceae | Sensitive fern family |
| Ophioglossaceae | Adder's-tongue family |
| Orchidaceae | Orchid family |
| Orobanchaceae | Broomrape family |
| Orthotrichaceae | |
| Osmundaceae | Royal Fern family |
| Oxalidaceae | Sorrel family |