Glossary of Terms and Concepts: O

Oil analysis – generic term for scientific examination of fluid properties, entrained wear and contamination for a variety of purposes. Selected goals and capabilities of oil analysis include:

GOAL / CAPABILITY	DESCRIPTION
Assay	Tests are performed to examine and determine a sample's characteristics, specifically related to the presence, absence or quantity of one or more substances. No conclusions are drawn past the quantitative results of the testing.
Specification	Selected performance-evaluating and assay tests are performed in order to compare a sample's properties to known standards. Requires specification reference data (acceptable minimum / maximum values, and 'typical' values) for each parameter measured. Conclusions are drawn based on the comparisons / rankings of the unknown sample to predetermined target ranges in order to assess quality and performance in service.
Identification	A combination of assay and specification tests are performed in order to determine a sample's manufacturing trade name and / or product designation, or to match an unknown sample to a known product. Requires advance knowledge of the specifications involved. Known references or chemical characteristics for each product may also be required if a complex mixture is to be separated by content, or if no specifications are available. Conclusions are drawn in terms of 'match, no-match, sample appears to be…,' with respect to the product in question.
Diagnosis	Assay, specification and identification tests are applied in conjunction with experience and statistics-based interpretation strategies, in order to assess the relationship between the result values and the fluid / mechanical condition of the system from which the sample was taken. Normally requires a time series of repeated measurements as well as individual requirements for the types of tests performed. Conclusions are drawn based on various maintenance and root cause analysis goals (see maintenance definition reference table).

Octane number -- term numerically indicating the relative antiknock value of a gasoline. For octane numbers 100 or below, it is based upon a comparison with the reference fuels isooctane (100 octane number) and n-heptane (0 octane number). The octane number of an unknown fuel is the percent by volume of isooctane with n-heptane which matches the unknown fuel in knocking tendencies under a specified set of conditions. Above 100, the octane number of a fuel is based on the engine rating, in terms of milliliters of tetraethyl lead in isooctane which matches that of the unknown fuel.
Oil drain interval -- time between complete oil drains from a sump or crankcase.
Oiliness -- that property of a lubricant that produces low friction under conditions of boundary lubrication. The lower the friction, the greater the oiliness.
Oil ring -- a loose ring, the inner surface of which rides a shaft or journal and dips into a reservoir of lubricant from which it carries the lubricant to the top of a bearing by its rotation with the shaft.
Oxidation -- the chemical combination of a substance with oxygen. All petroleum products are subject to oxidation, with resultant degradation of their composition and performance. The organic acids formed by oxidation are corrosive to metals. The process is accelerated by heat, light, metal catalysts and the presence of water, acids, or solid contaminants. The basic oxidation process for hydrocarbons is summarized as:

Organic peroxides form as the first reaction products;
Peroxides catalyze continued formation of alcohols, aldehydes, ketones, and organic acids;
Alcohols, etc. further oxidize to form high molecular-weight, oil insoluble polymers;
Polymers settle out as sludges, varnishes and gums.

Oxidation stability -- ability of a substance to resist natural degradation upon contact with oxygen.