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o'clock
the phrase "of the clock," used in English after a statement of time. It is fairly common in the works of Shakespeare. It could also be used as an angular measure by describing an angle in terms of the face of a standard (12-hour) clock. Each hour "o'clock" spans an angle of 30░, so "6 o'clock" means an angle of 180░ measured clockwise from dead ahead or any other point of reference.
octad
a unit of quantity equal to 8.
octane number
a measure used to express the ability of gasoline to reduce engine knocking. "Octane" is the name of a hydrocarbon whose molecules contain 8 carbon atoms and 18 hydrogen atoms, the 8 carbons being arranged in a long chain. Gasoline is compound containing hydrogen and carbon, Vehicles need to prevent knocking is not that octane but a different compound of 8 carbon atoms and 18 hydrogen atoms called iso-octane.  There is a similar unit, cetane number, used for rating diesel fuel.
octant
a unit of angle measure equal to 1/8 circle, 45░, or pi/4 radians. It is also a unit of solid angle measure, equal to 1/8 sphere, or pi/2 steradians, or about 5156.6 square degrees.
octave
a unit used in music to describe the ratio in frequency between notes. Two notes differ by one octave if the higher note has exactly twice the frequency of the lower one. It is derived from the Latin numeral 8 (octo ). In the Christian religious calendar, an octave is a period of 8 days beginning with a feast day and ending with the day one week after the feast day.
octavillo
an old Spanish unit of dry volume, equals about 289 milliliters (a little more than a cup), equivalent to about 17.64 cubic inches, 0.525 U.S. pint, or 0.5086 British imperial pint. Octavillo means "eighth"  in Spanish.
octennium
a unit of time equal to 8 years.
octet
a unit of quantity equal to 8. The name comes from the Latin numeral octo. It is also a unit of information equal to 8 bits. In most contexts, this is the same as a byte, but the byte can sometimes vary in size while the octet is always exactly 8 bits.
octuple, octuplet
a group of 8 items, especially 8 identical items; the word octuplet is also used for one member of the group.
odds
a statement of the probabilities that an event will or will not happen.
ohm
the SI unit of electric resistance, reactance, and impedance. The unit honors the German physicist Georg Simon Ohm (1787-1854). The capital Greek letter omega is used as the symbol for the ohm.

ohm is also a traditional German unit of liquid volume, no longer in use. It was equal to 1/6 fuder or about 150 liters (40 U.S. gallons).
oitavo
a traditional Portuguese unit of liquid volume, equals 1/32 fanega or about 1.73 liters (0.46 U.S. liquid gallon or 0.38 British imperial gallon).
oka or oke
a traditional unit of weight in Turkey and the eastern Mediterranean, equals 2.8 pounds or 1.28 kilograms. In Greece, the oka was standardized at 1282 grams. The oka was also used sometimes as a unit of liquid volume, representing the volume (roughly 1.25 liters) for water or wine.
okta
a unit of proportion equal to 1/8, it is from the Greek numeral 8, okto. It was coined to provide a word meaning "eighth" in all languages.
omer
a historic unit of volume for grains and dry commodities, mentioned in the Bible. The omer was equal to 0.1 ephah; It  equals about 4.032 liters, 246.05 cubic inches, 0.9154 U.S.dry gallon, or 0.8869 British imperial gallon.
once, oncia, onza
traditional names for the ounce unit in Romance languages. Tthe French once equals about 30.59 grams (1.079 ounce). The Italian oncia or onza is no longer used, but it equaled  about 27.3 grams (0.96 ounce). The Portuguese onša and Spanish onza about 28.69 grams (1.012 ounce).
ounce
The avoirdupois ounce, the unit commonly used in the United States, is 1/16 pound or about 28.3495 grams, approximately 0.911 457 troy ounce or 437.5 grains. It is derived from the Latin uncia, meaning a 1/12 part, because the Roman pound was divided into 12 ounces.

The troy ounce, used in pharmacy and jewelry, is 1/12 troy pound, 480 grains, or about 31.1035 grams. The troy ounce equals 192/175 = 1.09714 avoirdupois ounces. This unit is the traditional measure for gold and other precious metals; in particular, the prices of gold and silver quoted in financial markets are the prices per troy ounce. The troy ounce is divided into 20 pennyweight or into 8 troy drams. The troy ounce is sometimes abbreviated oz t or toz to distinguish it from the more common avoirdupois ounce oz.

The fluid ounce (fl oz), a traditional unit of liquid volume.
oxgang
an old English unit of land area equal to 1/8 hide or roughly 15 acres (6 hectares).

 

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