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  • Octane rating (octane number): a standard measure of the performance of a motor or aviation fuel. It is a measure of a fuel’s resistance to self ignition, hence a measure as well of the antiknock properties of the fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high-compression engines that generally have higher performance. In contrast, fuels with low octane numbers (but high cetane numbers) are ideal for diesel engines. Diesel fuel has a low octane rating, while gasoline and alcohol have high octane ratings suitable for spark-ignition engines.
  • Oil from tar sand: Synthetic crude oil (qv).
  • Oil mining: Application of a mining method to the recovery of bitumen.
  • Oilseed crops : Primarily soybeans, sunflower seed, canola, rapeseed, safflower, flaxseed, mustard seed, peanuts and cottonseed, used for the production of cooking oils, protein meals for livestock, and industrial uses.
  • Old growth: Timber stands with the following characteristics; large mature and over-mature trees in the overstory, snags, dead and decaying logs on the ground, and a multi-layered canopy with trees of several age classes.
  • One-pass Method:  A harvest practice where biomass and conventional roundwood (sawlogs) are harvested and recovered simultaneously.
  • On the Stump: Standing, uncut timber.
  • On/off fuel feed: A fuel feed system that delivers fuel to the grates on an intermittent basis in response to boiler water temperature and load variations. Efficient combustion is typically achieved during on cycles and during high-load conditions. In low-load conditions, and while off-cycle, combustion is less efficient.
  • OOIP (oil originally in place or originally oil in place): The quality of petroleum existing in a reservoir before oil recovery operation begin.
  • Open-loop biomass: Biomass that can be used to produce energy and bio-products even though it was not grown specifically for this purpose; include agricultural livestock waste, residues from forest-harvesting and crop-harvesting operation.
  • Organic compound: An organic compound contains carbon chemically bound to hydrogen. Organic compounds often contain other elements (particularly O, N, halogens, or S).
  • Organic residuals: Surplus or waste organic materials derived from biomass, including, but not necessarily limited to products, production and processing wastes and byproducts, remnants, bodily fluids, excrement or effluents derived from plants, animals, or microorganisms that have been previously used, processed, preprocessed, or remain after their primary production or use has occurred.
  • Other forest land: Forest land other than timberland and reserved forest land. It includes available forest land, which is incapable of annually producing 20 cubic feet per acre of industrial wood under natural conditions because of adverse site conditions such as sterile soils, dry climate, poor drainage, high elevation, steepness, or rockiness.
  • Other removals: Unutilized wood volume from cut or otherwise killed growing stock, from cultural operations such as precommercial thinnings, or from timberland clearing. Does not include volume removed from inventory through reclassification of timberland to productive reserved forest land.
  • Other sources: Sources of roundwood products that are not growing stock. These include salvable dead, rough and rotten trees, trees of noncommercial species, trees less than 5.0 inches d.b.h., tops, and roundwood harvested from non-forest land (for example, fence rows).
  • Output: The value of production by industry for a specific time period.
  • Oven Dry Ton:  An amount of wood that weighs 2,000 pounds at zero percent moisture content.
  • Over-fire air: Combustion air supplied above the grates and fuel bed. Also called secondary combustion air.
  • Over-stocked: Refers to forest stands where small trees and forest floor vegetation have grown in overly thick. This limits the ability of healthy trees to grow, prevents new trees from sprouting, and poses a significant wildfire hazard.
  • Overstory: The portion of the trees forming the uppermost canopy in a forest stand.
  • Oxidizing agent: a substance that removes electrons from another reactant in a redox chemical reaction.
  • Oxygenate: A substance which, when added to gasoline, increases the amount of oxygen in that gasoline blend; includes fuel ethanol, methanol, and methy1-tertiary-buty1 ether (MTBE).
  • Ozone (O3): or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2). Ozone in the lower atmosphere is an air pollutant with harmful effects on the respiratory systems of animals and will burn sensitive plants; however, the ozone layer in the upper atmosphere is beneficial, preventing damaging ultraviolet light from reaching the earth's surface. Ozone is a powerful oxidizing agent, far stronger than O2. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen (with a half-life of about half an hour in atmospheric conditions).

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