Oasis -a wet-point site in an arid area.

Obsequent stream -in a trellised drainage pattern, those streams flowing parallel but in the opposite direction to the consequent stream. The consequent stream follows the general direction of slope. The obsequent streams flow down the 'rear' side of the more resistant bands of rock which appear higher than the surrounding land.

Occluded front -see occlusion.

Occlusion -the coming together of the cold sectors of air in a depression as the warm sector is lifted from the surface altogether. Two kinds:

• cold occlusion -the rear cold sector is colder than the forward one and undercuts both the warm sector and the forward cold sector.

• warm occlusion -the forward cold sector is colder and the rear sector pushes the warm sector over the forward cold sector and then rises itself.

Ocean basin -the area of sea floor that is properly made up of oceanic crust i.e. not including areas of continental shelf .

Ocean current -large-scale movement of water within the oceans. Two kinds:   warm ocean current -runs near the surface from tropical areas to higher latitudes.    cold ocean current -runs deep along the ocean floor from higher latitudes to tropical areas.

Ocean currents are an important mechanism in the earth's heat transfer system from surplus areas to deficit areas. They have a profound influence on climate, making coastal areas near to warm currents far milder and wetter than their latitudinal position suggests they should be.

Oceanic crust - The outer layer of the Earth surface that lies beneath the oceans. As oceanic crust is denser than continental crust it generally lies below sea level.

Oceanic plate -a segment of the earth crust made up of sima. Found mostly, but not exclusively, beneath sea-level.

Ocean trench -deep depressions in the ocean floor (up to 11km) formed at a subduction zone where the denser plate is forced below the less dense one.

OECD -see Organisation for Economic Cooperation and Development.

Offshore - holds different meanings in different contexts:   geomorphology -features found on the seaward side of the wave breakpoint.   meteorology -wind moving from the land to the sea.

   economics - any economic activity conducted by a company or individual beyond the borders of their 'home' country i.e. where the company is incorporated or the individual is domiciled.

Oligopoly -supply of a good or service to a market is dominated and controlled by a handful of companies e.g. electricity, airlines. Under this scenario, collusion often plays a part in fixing prices and profits leading to inefficiencies similar to those of monopoly.

Omnivore -an organism that consumes plants ( producers) and animals (consumers).

Onshore -a movement from sea to land. Usually applied to wind but could also be used to describe movements of capital.

Oolith - a calcareous sphere, approximately 1 mm in diameter, formed by precipitation of concentric layers of calcium carbonate around a nucleus such as a grain of sand or shell fragment as it is rolled around by wave action in warm, shallow, tropical seas.     

Oolitic limestone - limestone formed from ooliths.       

OPEC -see Organisation of Oil Exporting Countries.

Opencast mining -extraction of a mineral deposit after first removing the layers of rock laid over it. Especially used when the mineral layer is horizontal or only gently dipping and the overlying material is shallow and/or fairly unconsolidated.

Open field system -a farming system developed in Anglo-Saxon culture. The typical example would feature a central village surrounded by three fields. Each year one filed would be left fallow and the other two cropped. The fields being worked were divided into strips, each strip being worked by a family from the village. Each family would have responsibility for multiple strips throughout the two fields being cropped. Livestock was then raised on common land between villages. This land was also the source of wood for building and fuel.

Optimiser -in economics, a theoretical decision-maker who has perfect knowledge of the market, which they use to come to a rational economic decision. While a necessary concept for the formation of economic models, this is also the main weakness as an optimiser cannot exist in the real world - although real people may aspire to be one.

Optimum population -the theoretical number of people required in a particular place at a particular time to maximise the return to each individual from all the available resources.

Ordinal data -data presented in order of importance or rank rather than actual values.

Organic farming -a modern farming system which deliberately eschews the use of chemical inputs to the farm. Has become increasingly popular in the EMDW in recent years, as people have demanded agricultural methods that are kinder to the environment. As well as having less negative impact on soils, plants and animals than conventional modern farming, the produce is better for human health. Does require more labour and often achieves lower yields therefore produce tends to be more expensive.

Organisation for Economic Cooperation and Development -established in 1961 by EMDW countries to coordinate aid to ELDW, discuss world growth and trade and to compile worldwide economic and social data to facilitate this. Full details at www.oecd.org

Organisation of Petroleum Exporting Countries -Algeria, Indonesia, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, UAE, and Venezuela. These oil producers run a cartel to control the world price of crude oil. OPEC was at the height of its power in the 1970s when it controlled 90% of the known world supplies. However, by maintaining high prices it forced other countries to conduct more rigorous oil exploration and allowed them to develop their own oil fields thus reducing its own importance in the world market. That said, it is still hugely influential and is the foundation stone of the geopolitical relationship between the Middle East and the rest of the world.

Organism -an individual living thing.

Orogenisis -mountain-building either by the accumulation of erupted material or by uplift due to tectonic movement.

Orographic -of mountains.

Output - relates to activity and means the amount of goods and services produced. In regeneration this would include the number of houses improved, the number of training places provided. It is mostly measured in physical or monetary units.

Outwash plain -a flat and gently sloping area of gravels, sands, silts and clays deposited beyond the snout of a glacier by meltwater issuing from it. Particles tend to be smoothed and rounded and size decreases with distance from the glacier.

Overgrazing -removal of vegetation cover from an area by grazing animals at a rate that exceeds the ability of the land to replace that vegetation.

Overland flow -water running over the surface of the land into a river channel or body of water. May occur due to: impermeable rock surfaces, saturated soils, rainfall intensity exceeding infiltration rate, snow/ice melt and compacted soils.

Overpopulation -when there are more than enough people to exploit all the resources of an area such that the marginal return to each person is lower than that of the optimal population.

Overproduction -usually applied to food yields exceeding the needs of the area in which they are produced. May be refined to mean yields exceeding both the consumption and export needs of the area in which they are produced.

Overspill -in human geography, those people who settle in a location beyond the boundaries of an urban area into which they tried to settle but could not find appropriate residences.

Ox-bow lake -a crescent-shaped lake formed when the main bend of a meander is cut off and becomes isolated from the main river channel.

Oxidation -a type of chemical weathering occurring when metal ions in a rock react with oxygen in the air or water.

Ozone layer -the layer is a concentration of ozone (O₃) in the stratosphere. Here it blocks harmful ultraviolet radiation. Ozone is also produced in the lower atmosphere due to pollution from car exhausts. Here it is unwelcome as it damages living organisms.