Oakum: shredded rope or hemp fiber impregnated with some form of light tar and used as a caulking or packing for joints.

Octane number: number indicating the knock rating or resistance to detonation of motor gasoline, defined as the percent by volume of isooctane (C8H18) in a mixture with n-heptane; this mixture has the same knock rating under standard engine test conditions as the test fuels.

Oil: common term applied to slippery liquids consisting of various

hydrocarbons; found in nature as petroleum, animal, vegetable or marine products, or synthesized in industry.

Oil-air lubrication: see air-oil lubrication.

Oil groove(s): frequently termed “oil ducts”; one or more grooves cut into the surface of bearing metal, the location and design of which are important for proper distribution of the lubricant. They prevent excessive oil loss from the bearing, serve as reservoirs to replenish lubricant supply and distribute the oil.

Oiliness: property of a lubricant that yields low friction under conditions of boundary lubrication, because of its affinity for metal surfaces. Polar compounds in the lubricant enhances this property, causing a physical adherence (absorbance) to the surfaces; the lower the friction, the greater the oiliness.

Oil mist or flog lubrication: oil atomized by compressed air, then conveyed by the air in a low- pressure distribution system to multiple points of lubricant application. At these points, as the mist flows through a nozzle of the proper type and size, it may condense as small quantities of liquid oil that lubricate the machine elements. The nozzles, or reclassifiers, include fog, spray mist and liquid fittings.

Oil pad: oil-saturated felt pad, generally used on plain bearings, that lubricates the rubbing surfaces; oil is supplied to the pad by wick, capillary, syphon, etc.

Oil ring: a loose ring, generally of greater in diameter than the shaft, the inner surface of which rides the shaft or journal, causing the ring to rotate; the ring dips into a reservoir from which it carries lubricant to the top of the shaft for distribution to a bearing.

Oil seal: one of many contact sealing devices used to reduce or eliminate oil leakage or to exclude foreign matter from a lubrication system (see lip seal). Oil seals are generally used on moving parts while gaskets seal non-moving parts such as housing (see gaskets).

Oil strainer: strainer that uses a metal screen or disc as the filtering medium; usually rated according to mesh size, not micrometer size.

Open gear lube: extremely heavy gear lubricants with viscosities in the asphaltic fluid range.

Organic acid: an organic compound with acid properties, obtained from such organic substances as animal, vegetable and mineral oils; for example, a fatty acid. Chemically, organic acids most often contain a carboxyl group (COOH). O-ring: an “O” –shaped automatic or squeeze-type packing manufactured from metallic or elastomeric materials, used for static and dynamic applications. The seal is compressed against the shaft or collar and energized by the pressure of the fluid being sealed to prevent leakage.

OSHA: Occupational Safety and Health Administration

Outboard bearing: one of two bearings supporting a shaft farthest from the drive unit. Some extend outside the machine, e.g., a shaft extended from a machine upon which is mounted a direct connected generator, pump, etc.

Oxidation: chemical process in which oxygen combines with another substance; enhanced by elevated temperature and the presence of a catalyst, such as copper, water or foreign matter. Oxidation of lubricants eventually produces acids and polymers, resulting in metal corrosion and sludge formation. Oxidation inhibitors function by interrupting the oxidation process at the first step, the formation of peroxides that serve as catalysts for the entire process (see inhibitors).

Oxidation stability: resistance of lubricants to chemical reaction with oxygen; several test methods are used.

Oxidation inhibitors : additives added in small quantities to petroleum and other products to retard oxidation reactions, increase shelf & service life. Also known as anti-oxidants. They function by interrupting the oxidation process at the first step, the formation of peroxides that serve as catalysts for the entire process (see inhibitors).