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- Nils Hjalmar Odhner
- William Erwood Old, Jr.
- Arthur Peter Hoblyn Oliver (1918–1984), from UK
- Walter Reginald Brook Oliver (1883–1957), from Australia/New Zealand
- Alcide d'Orbigny
- Arnold Edward Ortmann
Read more about this topic: Malacologist
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Read more about this topic: Malacologist