overzealous (adj.) Look up overzealous at Dictionary.com
also over-zealous, 1630s, from over- + zealous. Related: Overzealously; overzealousness.
oviparous (adj.) Look up oviparous at Dictionary.com
"producing eggs that are hatched outside the body of the female," 1640s, from Latin oviparus, from ovum "egg" (see egg (n.)) + stem of parere "to bring forth" (see pare).
ovulation (n.) Look up ovulation at Dictionary.com
1848, from Modern Latin ovulum (see ovule) + -ation.
ovule (n.) Look up ovule at Dictionary.com
1821, from French ovule and directly from Modern Latin ovulum, literally "small egg," diminutive of Latin ovum "egg" (see ovum).
ovum (n.) Look up ovum at Dictionary.com
(plural ova), 1706, from Latin ovum "egg," cognate with Greek oon, Old Norse egg, Old English æg, all perhaps from PIE root *awi- (see egg (n.)).
owe (v.) Look up owe at Dictionary.com
Old English agan (past tense ahte) "to have, own," from Proto-Germanic *aigan "to possess" (source also of Old Frisian aga, Old Norse eiga, Old High German eigan, Gothic aigan "to possess, have"), from PIE *aik- "to be master of, possess" (source also of Sanskrit ise "he owns," isah "owner, lord, ruler;" Avestan is- "riches," isvan- "well-off, rich").

Sense of "to have to repay" began in late Old English with the phrase agan to geldanne literally "to own to yield," which was used to translate Latin debere (earlier in Old English this would have been sceal "shall"); by late 12c. the phrase had been shortened to simply agan, and own (v.) took over this word's original sense.

An original Germanic preterite-present verb (along with can (v.1), dare, may, etc.). New past tense form owed arose 15c. to replace oughte, which developed into ought (v.).
Owen Look up Owen at Dictionary.com
Celtic masc. proper name, ultimately from Greek eugenes "well-born;" via Gaelic Eòghann, Old Irish Eogán, Old Welsh Eugein, Ougein. In Medieval records, frequently Latinized as Eugenius; the form Eugene emerged in Scotland by late 12c. The Breton form Even led to modern French Ivain. Owenite in reference to the communistic system of social reformer Robert Owen (1771-1858) is attested from 1829.
owl (n.) Look up owl at Dictionary.com
Old English ule "owl," from Proto-Germanic *uwwalon- (source also of Middle Dutch, Dutch uil, Old High German uwila, German Eule, Old Norse ugla), a diminutive of PIE root *u(wa)l-, which is imitative of a wail or an owl's hoot (compare Latin ulula "owl;" also see ululation). The bird was employed proverbially and figuratively in reference to nocturnal habits, ugliness, and appearance of gravity and wisdom (often ironic).
own (adj.) Look up own at Dictionary.com
Old English agen "one's own," literally "possessed by," from Proto-Germanic *aigana- "possessed, owned" (source also of Old Saxon egan, Old Frisian egin, Old Norse eiginn, Dutch eigen, German eigen "own"), from past participle of PIE *aik- "to be master of, possess," source of Old English agan "to have" (see owe).
own (v.) Look up own at Dictionary.com
c. 1200, ouen, "to possess, have; rule, be in command of, have authority over;" from Old English geagnian, from root agan "to have, to own" (see owe), and in part from the adjective own (q.v.). It became obsolete after c. 1300, but was revived early 17c., in part as a back-formation of owner (mid-14c.), which continued. From c. 1300 as "to acknowledge, admit as a fact," said especially of things to one's disadvantage. To own up "make full confession" is from 1853. Related: Owned; owning.
ox (n.) Look up ox at Dictionary.com
Old English oxa "ox" (plural oxan), from Proto-Germanic *ukhson (source also of Old Norse oxi, Old Frisian oxa, Middle Dutch osse, Old Saxon, Old High German ohso, German Ochse, Gothic auhsa), from PIE *uks-en- "male animal," (source also of Welsh ych "ox," Middle Irish oss "stag," Sanskrit uksa, Avestan uxshan- "ox, bull"), said to be from root *uks- "to sprinkle," related to *ugw- "wet, moist." The animal word, then, is literally "besprinkler."
Oxford Look up Oxford at Dictionary.com
university town in England, Middle English Oxforde, from Old English Oxnaforda (10c.) literally "where the oxen ford." In reference to a type of shoe laced over the instep, it is attested from 1721 (Oxford-cut shoes). Related: Oxfordian; Oxfordish; Oxfordist; Oxfordy.
oxide (n.) Look up oxide at Dictionary.com
"compound of oxygen with another element," 1790, from French oxide (1787), coined by G. de Morveau and A. Lavoisier from ox(ygène) (see oxygen) + (ac)ide "acid" (see acid).
Oxonian (adj.) Look up Oxonian at Dictionary.com
"pertaining to Oxford or to Oxford University," 1640s, from Medieval Latin oxonia, Latinized form of Middle English Oxforde (see Oxford). Earlier as a noun (1540s).
oxygen (n.) Look up oxygen at Dictionary.com
gaseous chemical element, 1790, from French oxygène, coined in 1777 by French chemist Antoine-Laurent Lavoisier (1743-1794), from Greek oxys "sharp, acid" (see acrid) + 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.
oxymoron (n.) Look up oxymoron at Dictionary.com
1650s, from Greek oxymoron, noun use of neuter of oxymoros (adj.) "pointedly foolish," from oxys "sharp" (see acrid) + moros "stupid" (see moron). Rhetorical figure by which contradictory terms are conjoined so as to give point to the statement or expression; the word itself is an illustration of the thing. Now often used loosely to mean "contradiction in terms." Related: Oxymoronic.
oyer (n.) Look up oyer at Dictionary.com
early 15c., "a hearing of causes," from Anglo-French oyer, Old French oir, from Latin audire "to hear" (see audience). 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 U.S. a higher criminal court.
oyez (interj.) Look up oyez at Dictionary.com
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" (Anglo-French oier; see audience).
oyster (n.) Look up oyster at Dictionary.com
early 14c., from Old French oistre (Modern French huître), from Latin ostrea, plural or fem. of ostreum "oyster," from Greek ostreon, from PIE *ost- "bone" (see osseous). Related to Greek ostrakon "hard shell" and to osteon "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]
Ozark Look up Ozark at Dictionary.com
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.
ozone (n.) Look up ozone at Dictionary.com
1840, from German Ozon, coined in 1840 by German chemist Christian Friedrich Schönbein (1799-1868) from Greek ozon, neuter present participle of ozein "to smell" (see odor). So called for its pungent odor.