O
- Octane
- Odd Hassel
- ohm
- Olivine
- Opal
- Optical isomerism
- Organic chemistry
- organic compound
- Organic nomenclature
- Organic reaction
- Organic synthesis
- Organolithium reagent
- Orthoclase
- Osmium
- Osmium tetroxide
- Otto Hahn
- Otto Paul Hermann Diels
- Otto Wallach
- Oxidation
- Oxidation number
- oxidation state
- oxide
- oxygen
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