O |
| Omnivore | Organisms that feed both on autotrophs and other heterotrophs. |
| Open access resource | A resource to which no control on access is applied; therefore, the resource can be accessed by any individual at any time. |
| Opportunity costs | When economists refer to the “opportunity cost” of a resource, they mean the value of the next-highest-valued alternative use of that resource. http://www.econlib.org/library/Enc/OpportunityCost.html |
| Option value | The expected value of future information from or about natural resources. Conserving resources today maintains the option of resource value in the future. |
| Orbit | The path of a body or particle under the influence of a gravitational or other force. For instance, to go around the Earth or other body in an orbit. |
| Organic | 1. Relating to an organism. 2. Derived from an organism. |
| Organism | Any form of life. |
| Orographic rainfall | Occurs when warm, moist air is forced to rise over elevated land. The air is cooled as it rises, leading to condensation, cloud formation, and rainfall. |
| Over-exploitation | Exploitation to the point of diminishing returns. Overexploitation of natural resources — through unsustainable hunting, fishing, or extracting raw material — has serious implications for biodiversity. The social costs of overexploitation are high, leaving communities with little alternatives for employment and possibly even food. |
| Overgrazing | Occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. |
| Ownership (as defined in the Paris Declaration) | The Paris Declaration of March 2005 on the effectiveness of development cooperation establishes ownership as one of the fundamental principles of cooperation. Ownership as a core principle means that the countries receiving development aid are primarily responsible for the initiatives implemented in the context of development cooperation. |
