| Substance | Abbreviation | Systematics | Provings |
| Obsidianum Obsidian |
Lap-ob | Oxida amorpha |
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| Ochna madagascariensis |
Ochn-m | Ochnaceae |
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| Ochna serrulata Mall-leaved plane |
Ochn-s | Ochnaceae |
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| Ocimum basilicum Basil |
Oci-b | Labiatae/ Lamiaceae |
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| Ocimum canum/ Basilicum album Brazilian Alfavaca |
Oci | Labiatae/ Lamiaceae |
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| Ocimum sanctum Sacred Basil |
Oci-s | Labiatae/ Lamiaceae |
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| Octoknema okoubaka / Okoubaka aubrevillei Okoubaka |
Okou | Santalaceae |
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