O1—Lunar diurnal constituent. See K1. Speed = T – 2s + h = 13.943,035,6° per solar hour.

obliquity factor—A factor in an expression for a constituent tide (or tidal current) involving the angle of the inclination of the Moon's orbit to the plane of the Earth's Equator.

obliquity of the ecliptic—The angle which the ecliptic makes with the plane of the Earth's Equator. Its value is approximately 23.45°.

obliquity of the Moon's orbit—The angle which the Moon's orbit makes with the plane of the Earth's Equator. Its value varies from 18.3° to 28.6°, depending upon the longitude of the Moon's ascending node; the smaller value corresponding to a longitude of 180° and the larger one, to a longitude of 0°.

oceanography—Oceanography is the science of all aspects of the oceans, in spite of its etymology. The term, oceanography, implies the interrelationships of the various marine sciences of which it is composed. This connotation has arisen through the historical development of marine research in which it has been found that a true understanding of the oceans is best achieved through investigations based on the realization that water, its organic and inorganic contents, motions, and boundaries are mutually related and interdependent.

ordinary—With respect to tides, the use of this nontechnical word has, for the most part, been determined to be synonymous with mean. Thus, ordinary high (low) water is the equivalent of mean high (low) water. The use of ordinary in tidal terms is discouraged. orifice—See stilling well and protective well.

overfalls—Breaking waves caused by the meeting of currents or by waves moving against the current. See rip.

overtide—A harmonic tidal (or tidal current) constituent with a speed that is an exact multiple of the speed of one of the fundamental constituents derived from the development of the tide-producing force. The presence of overtides is usually attributed to shallow water conditions. The overtides usually considered in tidal work are the harmonics of the principal lunar and solar semidiurnal constituents M2 and S2, and are designated by the symbols M4, M6, M8, S4, S6, etc. The magnitudes of these harmonics relative to those of the fundamental constituents are usually greater in the tidal current than in the tide.

Oyashio—A current setting southwestward along the Siberian, Kamchatka, and Kuril Islands coasts in the Bering Sea and North Pacific Ocean.