Etymology
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oxygen (n.)

gaseous chemical element, 1790, from French oxygène, coined in 1777 by French chemist Antoine-Laurent Lavoisier (1743-1794), from Greek oxys "sharp, acid" (from PIE root *ak- "be sharp, rise (out) to a point, pierce") + French -gène "something that produces" (from Greek -genes "formation, creation;" see -gen).

Intended to mean "acidifying (principle)," it was a Greeking of French principe acidifiant. So called because oxygen was then considered essential in the formation of acids (it is now known not to be). The element was isolated by Priestley (1774), who, using the old model of chemistry, called it dephlogisticated air. The downfall of the phlogiston theory required a new name, which Lavoisier provided. Oxygen-mask is attested from 1912.

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oxymoron (n.)

in rhetoric, "a figure conjoining words or terms apparently contradictory so as to give point to the statement or expression," 1650s, from Greek oxymōron, noun use of neuter of oxymōros (adj.) "pointedly foolish," from oxys "sharp, pointed" (from PIE root *ak- "be sharp, rise (out) to a point, pierce") + mōros "stupid" (see moron). The word itself is an illustration of the thing. Now often used loosely to mean "contradiction in terms." Related: Oxymoronic.

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oy (interj.)

Yiddish exclamation of dismay, 1892, American English. Extended form oy vey (1959) includes Yiddish vey, from German Weh "woe" (see woe).

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oyer (n.)

early 15c., "a criminal hearing of causes," from Anglo-French oyer, Old French oir, oier, from Latin audire "to hear" (from PIE root *au- "to perceive"). Especially in phrase oyer and terminer (early 15c., but from late 13c. in Anglo-Latin and Anglo-French), literally "a hearing and determining," in England a court of judges of assize, in some U.S. states a higher criminal court.

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oyez (interj.)

a call for silence and attention; the introduction to a proclamation made by an officer of a law-court," early 15c., from Anglo-French oyez "hear ye!" (late 13c., Old French oiez), a cry uttered (usually thrice) to call attention, from Latin subjunctive audiatis, plural imperative of audire "to hear" (from PIE root *au- "to perceive").

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oyster (n.)

"edible bivalve mollusk of the family Ostreidæ," late 13c., oistre, from Old French oistre, uistre (Modern French huître) and directly from Latin ostrea, plural or fem. of ostreum "oyster," from Greek ostreon, from PIE root *ost- "bone." It is thus related to Greek ostrakon "a hard shell" and to osteon "a bone." The h- in the modern French word is a regular development; compare huile "oil" (Latin oleum), huit "eight" (Latin octo).

Why then the world's mine Oyster, which I, with sword will open. [Shakespeare, "The Merry Wives of Windsor," II.ii.2]

Oyster-bed "place where oysters breed or are bred" is from c. 1600; oyster-knife, used for opening oysters, is recorded from 1690s. Oysterman "man engaged in rearing, taking, or selling oysters" is attested from 1550s. The common statement that edible oysters are in season only in months that end in -r is recorded by 1765.

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oz. 
abbreviation of ounce (n.1), 1540s, from Italian oz. (15c.), abbreviation of onza.
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Oz 
mythical land in L. Frank Baum's "The Wonderful Wizard of Oz" (1900) and sequels; according to an anecdote written by Baum in 1903, inspired by a three-drawer desktop cabinet letter file, the last drawer labeled O-Z. As Australian slang for "Australia," attested by 1983.
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Ozark 
mountains of southcentral United States, said to be from French aux Arcs, short for aux Arkansas "to the Arkansas (Indians)," who once inhabited that region. See Arkansas.
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