- Obsidian
- Ocean Circulation and currents
- Ocean Trenches
- Oceanography
- Oceans and Seas
- Odén, Svante N. F.(1924-1986)
- Offshore Bars
- Oligocene Epoch
- Olivine
- Ophiolite Suites
- Ordovician Period
- Orientation of Strata
- Origin of Life
- Orogeny
- Ortelius, Abraham (1527-1598)
- Owen, Sir Richard (1804-1892)
- Oxidation-Reduction Reaction
- Oxygen
- Ozone
- Ozone Layer and Ozone Hole Dynamics
- Ozone Layer Depletion
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