Contents: Foreword. Preface. Introduction. 1. Ochna. 2. Ocimum. 3. Odina. 4. Oenanthe. 5. Oenothera. 6. Olax. 7. Oldenlandia. 8. Olea. 9. Oleandra. 10. Oligochaeta. 11. Ononis. 12. Onosma. 13. Onychium. 14. Operculina. 15. Ophioglossum. 16. Ophiopogon. 17. Ophiorrhiza. 18. Oplismenus. 19. Opuntia. 20. Orchis. 21. Oreocnide. 22. Origanum. 23. Ormocarpum. 24. Orobanche. 25. Orophea. 26. Oroxylum. 27. Orthosiphon. 28. Orygia. 29. Oryza. 30. Osbeckia. 31. Osmanthus. 32. Ostodes. 33. Osyris. 34. Otochilus. 35. Ougeinia. 36. Oxalis. 37. Oxyria. 38. Oxystelma. 39. Oxytenanthera. Appendices: 1. List of plants reported for only ethnobotanical/traditional uses. 2. List of plants not included in the volume due to change in botanical nomenclature. 3. List of some important books, treatises and encyclopedias referred. Indices: 1. Index of family names including generic names. 2. Index of botanical names. 3. Index of chemical constituents. 4. Index of pharmacological and biological activity. 5. Index of regional and other names. Plant Genera Included in Earlier Volumes: List of Plant Genera Vols.1 to 17 (Abe – Ny).