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OAPEC: Abbreviation for Organization of Arab Petroleum Exporting Countries founded in 1968 for cooperation in economic and petroleum affairs. See OPEC.

Obligation To Serve: The obligation of a utility to provide electric service to any customer who seeks that service, and is willing to pay the rates set for that service. Traditionally, utilities have assumed the obligation to serve in return for an exclusive monopoly franchise.

Occupancy Sensor: A control device that senses the presence of a person in a given space, commonly used to control lighting systems in buildings.

Octane: A flammable liquid hydrocarbon found in petroleum. Octane is used as a standard or rating scale to measure the anti–knock properties of motor fuel. Octane is also any of several isometric liquid paraffin hydrocarbons, C8H18. Normal octane is a colorless liquid found in petroleum boiling at 124.6 degrees Celsius.

Octane Rating: The octane rating is the measure of a gasoline’s resistance to exploding too early in the engine cycle, which causes knocking. In other words, a number used to indicate gasoline's antiknock performance in motor vehicle engines. The higher the octane rating, the lower the chance of premature ignition. The two recognized laboratory engine test methods for determining the antiknock rating, i.e., octane rating, of gasoline grades are the Research method and the Motor method. To provide a single number as guidance to the consumer, the antiknock index (R + M)/2, which is the average of the Research and Motor octane numbers, was developed.

Off–Road: Any non–stationary device, powered by an internal combustion engine or motor, used primarily off the highways to propel, move or draw persons or property, and used in any of the following applications: marine vessels, construction/farm equipment, locomotives, utility and lawn and garden equipment, off–road motorcycles and off–highway vehicles.

Ohm: A unit of measure of electrical resistance. One volt can produce a current of one ampere through a resistance of one ohm.

Oil Shale: A type of rock containing organic matter that produces large amounts of oil when heated to high temperatures.

Oligopoly: A few sellers who exert market control overprices.

OPEC: Abbreviation for Organization of Petroleum Exporting Countries founded in 1960 to unify and coordinate petroleum polices of the members. The headquarters of OPEC is in Vienna, Austria.

Orientation: the position of a building relative to the points of a compass.

Original Equipment Manufacturer (OEM): Refers to the manufacturers of complete vehicles or heavy‚Äöduty engines, as contrasted with remanufacturers, converters, retrofitters, up–fitters and repowering or rebuilding contractors who are overhauling engines, adapting or converting vehicles or engines obtained from the OEMs, or exchanging or rebuilding engines in existing vehicles.

Orphaned Well: Stripper oil wells that are not producing, have not been plugged, and whose owners are either insolvent or cannot be located.

Outage (Electric Utility): An interruption of electric service that is temporary (minutes or hours) and affects a relatively small area (buildings or city blocks). See Blackout.

Outer Continental Shelf (OCS): The submerged lands extending from the outer limit of the historic territorial sea (typically three miles) to some undefined outer limit, usually a depth of 600 feet. In the United States, this is the portion of the shelf under federal jurisdiction. See Continental Shelf.

Outside Air: Air taken from outdoors and not previously circulated through the HVAC system.

Overhang: Any horizontal projection that serves as a shading element for a window.

Oxides Of Nitrogen: See NOx.

Oxygenate: Oxygenate is a term used in the petroleum industry to denote octane components containing hydrogen, carbon, and oxygen in their molecular structure. Oxygenates are substances which, when added to gasoline, increase the amount of oxygen in that gasoline blend. The increased oxygen content given by oxygenates promotes more complete combustion, thereby reducing tailpipe emissions. An oxygenate is a prime ingredient in reformulated gasoline. Common oxygenates include ethers such as Methyl Tertiary Butyl Ether (MTBE) and Ethyl Tertiary Butyl Ether (ETBE) and alcohols such as ethanol or methanol.

Ozone: A kind of oxygen that has three atoms per molecule instead of the usual two. Ozone is a poisonous gas, but the ozone layer in the upper atmosphere shields life on earth from deadly ultraviolet radiation from space. The molecule contains three oxygen atoms (O3).