| Id. | Group | Links |
| 133 | Oenotheraceae/ Onagraceae | |
| 177 | Oils and similar Compounds of Carbon | |
| 129 | Oleaceae | |
| 263 | Oniscidae | |
| 143 | Orchidaceae | |
| 171 | Orobanchaceae | |
| 215 | Orthomyxoviridae | |
| 264 | Osmium | |
| 108 | Ostreidae | |
| 130 | other industrial products | |
| 339 | Other Phenomena | |
| 33 | Oxygenium | |