| Observation Well |
United Nations, Glossary of Environment Statistics |
| Occupational Health Hazards |
United Nations, Glossary of Environment Statistics |
| Ocean Dumping |
United Nations, Glossary of Environment Statistics |
| Odor Threshold |
United States Environmental Protection Agency, Office of Compliance and Assurance |
| Odor Threshold |
United States Environmental Protection Agency, Terms of Environment |
| OECD Guidelines |
United States Environmental Protection Agency, Terms of Environment |
| Oil and Gas Waste |
United States Environmental Protection Agency, Terms of Environment |
| Oil Dark |
United Nations, Glossary of Environment Statistics |
| Oil Desulfurization |
United States Environmental Protection Agency, Terms of Environment |
| Oil Fingerprinting |
United Nations, Glossary of Environment Statistics |
| Oil Spill |
United States Environmental Protection Agency, Terms of Environment |
| Oil Spill |
United Nations, Glossary of Environment Statistics |
| Oligotrophic Lakes |
United States Environmental Protection Agency, Terms of Environment |
| Onchocerciasis |
United Nations, Glossary of Environment Statistics |
| Oncogenic |
United Nations, Glossary of Environment Statistics |
| Onconogenicity |
United States Environmental Protection Agency, Terms of Environment |
| One-hit Model |
United States Environmental Protection Agency, Terms of Environment |
| Opacity |
United States Environmental Protection Agency, Terms of Environment |
| Opacity |
United Nations, Glossary of Environment Statistics |
| Oral Toxicity |
United States Environmental Protection Agency, Terms of Environment |
| Organic |
United States Environmental Protection Agency, Terms of Environment |
| Organic Chemicals/Compounds |
United States Environmental Protection Agency, Terms of Environment |
| Organic Compounds |
United Nations, Glossary of Environment Statistics |
| Organic Farming |
United Nations, Glossary of Environment Statistics |
| Organic Fertilizers |
United Nations, Glossary of Environment Statistics |
| Organic Matter |
United States Environmental Protection Agency, Terms of Environment |
| Organically Grown |
United States Environmental Protection Agency, Office of Compliance and Assurance |
| Organism |
United States Environmental Protection Agency, Office of Compliance and Assurance |
| Organism |
United States Environmental Protection Agency, Terms of Environment |
| Organism |
United Nations, Glossary of Environment Statistics |
| Organophosphates |
United States Environmental Protection Agency, Terms of Environment |
| Organophosphates |
United Nations, Glossary of Environment Statistics |
| Organotins |
United States Environmental Protection Agency, Terms of Environment |
| Osmosis |
United States Environmental Protection Agency, Terms of Environment |
| Osmosis |
United Nations, Glossary of Environment Statistics |
| Overburden |
United States Environmental Protection Agency, Terms of Environment |
| Overdraft |
United States Environmental Protection Agency, Terms of Environment |
| Overpumping |
United Nations, Glossary of Environment Statistics |
| Oxidant |
United States Environmental Protection Agency, Office of Compliance and Assurance |
| Oxidant |
United States Environmental Protection Agency, Terms of Environment |
| Oxidant |
United Nations, Glossary of Environment Statistics |
| Oxidation |
United States Environmental Protection Agency, Terms of Environment |
| Oxidation |
United Nations, Glossary of Environment Statistics |
| Oxidation Pond |
United States Environmental Protection Agency, Terms of Environment |
| Oxidation Pond |
United Nations, Glossary of Environment Statistics |
| Oxygenated Fuels |
United States Environmental Protection Agency, Terms of Environment |
| Ozonation/Ozonator |
United States Environmental Protection Agency, Terms of Environment |
| Ozone |
United States Environmental Protection Agency, Office of Compliance and Assurance |
| Ozone (O3) |
United Nations, Glossary of Environment Statistics |
| Ozone Depletion |
United States Environmental Protection Agency, Terms of Environment |
| Ozone Depletion |
United Nations, Glossary of Environment Statistics |
| Ozone Hole |
United States Environmental Protection Agency, Terms of Environment |
| Ozone Hole |
United Nations, Glossary of Environment Statistics |
| Ozone Layer |
United States Environmental Protection Agency, Terms of Environment |
| Ozone Layer |
United Nations, Glossary of Environment Statistics |
| Ozonosphere |
United Nations, Glossary of Environment Statistics |