Online Etymology Dictionary

o: interjection of fear, surprise, admiration, etc.; see oh.
O: blood type, 1926, originally "zero," denoting absence of A and B agglutinogens.
O': as a prefix in Irish names, from Irish ó, ua (Old Irish au) "descendant."
o'clock (adj.): c. 1720, abbreviation of of the clock (1640s), from Middle English of the clokke (late 14c.). Use of clock hand positions to describe vector directions or angles is from late 18c.
o'er: poetic contraction of over.
O.D.: abbreviation of overdose, attested from 1960.
oaf (n.): 1620s, auf, oph (modern form from 1630s), "a changeling; a foolish child left by the fairies" [Johnson], from a Scandinavian source such as Norwegian alfr "silly person," in Old Norse "elf" (see elf). Hence, "a misbegotten, deformed idiot." Until recently, some dictionaries still gave the plural as oaves.
oafish (adj.): 1610s, from oaf + -ish. Related: Oafishly; oafishness.
oak (n.): Old English ac "oak tree," from Proto-Germanic *aiks (source also of Old Norse eik, Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche), of uncertain origin with no certain cognates outside Germanic.

The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.); likewise in Greek and Celtic words for "oak" are from the Indo-European root for "tree," probably reflecting the importance of the oak to ancient Indo-Europeans. The Old Norse form was eik, but as there were no oaks in Iceland the word came to be used there for "tree" in general. Used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words.
oaken (adj.): "made of, or consisting of, oak," late 12c., from oak + -en (2).
oakum (n.): "loose fiber obtained from taking apart old hemp ropes," early 15c., from Old English acumba "tow, oakum, flax fibers separated by combing," literally "what is combed out," from Proto-Germanic *us-kambon (source of Old High German achambi); first element cognate with Old English a- "away, out, off;" second element from stem of cemban "to comb," from camb "a comb;" from PIE *gembh- "tooth, nail" (see comb (n.)).
oar (n.): Old English ar "oar," from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), of unknown origin; perhaps related to Latin remus "oar," Greek eretes "rower," eretmos "oar."
oarlock (n.): mid-14c., from oar + lock (n.1).
oasis (n.): 1610s, from French oasis (18c.) and directly from Late Latin oasis, from Greek oasis, probably from Hamitic (compare Coptic wahe, ouahe "oasis," properly "dwelling place," from ouih "dwell"). The same Egyptian source produced Arabic wahah.
oat (n.): Old English ate (plural atan) "grain of the oat plant, wild oats," of uncertain origin, possibly from Old Norse eitill "nodule," denoting a single grain, of unknown origin. The English word has cognates in Frisian and some Dutch dialects. Famously defined by Johnson as, "A grain, which in England is generally given to horses, but in Scotland supports the people."

The usual Germanic name is derived from Proto-Germanic *khabran (source also of Old Norse hafri, Dutch haver, source of haversack). Wild oats, "crop that one will regret sowing," is first attested 1560s, in reference to the folly of sowing these instead of good grain.
That wilfull and vnruly age, which lacketh rypenes and discretion, and (as wee saye) hath not sowed all theyr wyeld Oates. [Thomas Newton, "Lemnie's Touchstone of complexions," 1576]

Fred: I still want to sow some wild oats!
Lamont: At your age, you don't have no wild oats, you got shredded wheat.
["Sanford and Son"]
Hence, to feel (one's) oats "be lively," 1831, originally American English.
oater (n.): Western film, "horse opera," 1946, from oat, as the typical food of horses.
oath (n.): Old English að "oath, judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aithaz (source also of Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (source also of Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure. In reference to careless invocations of divinity, from late 12c.
oatmeal (n.): late 14c., from oat + Middle English mele (see meal (n.2)).
ob-: prefix meaning "toward, against, across, down," also used as an intensive, from Latin ob "toward, to, over against, in the way of, by reason of, about, before, in front of," from PIE root *epi, also *opi "near, against" (see epi-).
Obadiah: masc. proper name, fourth of the Twelve Prophets of the Old Testament, from Hebrew Obhadyah, literally "servant of the Lord," from abhadh "he served, worshipped," related to Arabic 'abada "he served," 'abd "slave, worshipper."
obbligato (adj.): musical instruction, 1724, from Italian obbligato, literally "obligated," from Latin obligatus, past participle of obligare "to bind" (see oblige). In reference to a necessary accompaniment by a single instrument.
obduracy (n.): "stubbornness," 1590s, from obdurate + -cy.
obdurate (adj.): mid-15c., "stubborn; hardened," from Latin obduratus "hardened," past participle of obdurare "be hard, hold out, persist, endure," from ob "against" (see ob-) + durare "harden, render hard," from durus "hard" (see endure). Related: Obdurately.
obduration (n.): c. 1400, "hard-heartedness," from Latin obdurationem (nominative obduratio), noun of state from past participle stem of obdurare (see obdurate).
obeah (n.): "sorcery, witchcraft" among blacks in Africa and the W.Indies, 1760, from a West African word, such as Efik (southern Nigeria) ubio "a thing or mixture left as a charm to cause sickness or death," Twi ebayifo "witch, wizard, sorcerer."
obedience (n.): c. 1200, "submission to a higher power or authority," from Old French obedience "obedience, submission" (12c.) and directly from Latin oboedientia "obedience," noun of quality from oboedientem (nominative oboediens); see obedient. In reference to dog training from 1930.
obedient (adj.): c. 1200, from Old French obedient "obedient" (11c.), from Latin oboedientem (nominative oboediens), present participle of oboedire "to obey" (see obey). Related: Obediently.
obeisance (n.): late 14c., "act or fact of obeying," from Old French obeissance "obedience, service, feudal duty" (13c.), from obeissant, present participle of obeir "obey," from Latin oboedire (see obey). Sense in English altered late 14c. to "bending or prostration of the body as a gesture of submission or respect" by confusion with abaisance. Related: Obeisant.
obelisk (n.): "rectangular stone column tapering at the top," 1560s, from Middle French obélisque (16c.) and directly from Latin obeliscus "obelisk, small spit," from Greek obeliskos "small spit, obelisk, leg of a compass," diminutive of obelos "a spit, pointed pillar, needle." Related: Obeliskine.
Oberon: king of the faeries and husband of Titania in medieval lore, from French Obéron, from Old French Auberon, perhaps from a Germanic source related to elf.
obese (adj.): 1650s, back-formation from obesity and in part from Latin obesus "fat, stout, plump," past participle of obedere "that has eaten itself fat" (see obesity). According to OED, "Rare before 19th c." Related: Obeseness. Latin obesus was translated in Old English as oferfæt "overfat."
obesity (n.): 1610s, from French obésité and directly from Latin obesitas "fatness, corpulence," from obesus "that has eaten itself fat," past participle of obdere "to eat all over, devour," from ob "over" (see ob-) + edere "eat" (see edible).
obey (v.): late 13c., from Old French obeir "obey, be obedient, do one's duty" (12c.), from Latin obedire, oboedire "obey, be subject, serve; pay attention to, give ear," literally "listen to," from ob "to" (see ob-) + audire "listen, hear" (see audience). Same sense development is in cognate Old English hiersumnian. Related: Obeyed; obeying.
obfuscate (v.): 1530s, from Latin obfuscatus, past participle of obfuscare "to darken," from ob "over" (see ob-) + fuscare "to make dark," from fuscus "dark" (see dusk). Related: Obfuscated; obfuscating.
obfuscation (n.): early 15c., originally medical, "the darkening of a sore," from Latin obfuscationem (nominative obfuscatio), noun of action from past participle stem of obfuscare (see obfuscate).
Obie: one of the annual awards given to off-Broadway theater, 1967, from O.B. as the abbreviation of Off-Broadway.
obit (n.): late 14c., "death," from Middle French obit or directly from Latin obitus "death," noun use of past participle of obire "to die," literally "to go toward" (see obituary). In modern usage (since 1874) it is usually a clipped form of obituary, though it had the same meaning of "published death notice" 15c.-17c. The scholarly abbreviation ob. with date is from Latin obiit "(he) died," third person singular of obire.
obiter (adv.): Latin, literally "by the way," from ob "to, toward" (see ob-) + iter "journey" (see ion). Klein's sources, however, say it is ob- with the suffix -iter in analogy of circiter "about" from circa. Also see obituary.
obiter dictum: "statement in passing," a judge's expression of opinion not regarded as binding or decisive, Latin, literally "something said incidentally;" see obiter + dictum.
obituary (n.): 1706, "register of deaths," from Medieval Latin obituarius "a record of the death of a person," literally "pertaining to death," from Latin obitus "departure, a going to meet, encounter" (a euphemism for "death"), from stem of obire "go toward, go to meet" (as in mortem obire "meet death"), from ob "to, toward" (see ob-) + ire "to go" (see ion). Meaning "record or announcement of a death, especially in a newspaper, and including a brief biographical sketch" is from 1738. As an adjective from 1828. A similar euphemism is in Old English cognate forðfaran "to die," literally "to go forth;" utsið "death," literally "going out, departure."
object (n.): late 14c., "tangible thing, something perceived or presented to the senses," from Medieval Latin objectum "thing put before" (the mind or sight), noun use of neuter of Latin obiectus "lying before, opposite" (as a noun in classical Latin, "charges, accusations"), past participle of obicere "to present, oppose, cast in the way of," from ob "against" (see ob-) + iacere "to throw" (see jet (v.)). Sense of "thing aimed at" is late 14c. No object "not a thing regarded as important" is from 1782. As an adjective, "presented to the senses," from late 14c. Object lesson "instruction conveyed by examination of a material object" is from 1831.
object (v.): c. 1400, "to bring forward in opposition," from Old French objecter and directly from Latin obiectus, past participle of obiectare "to cite as grounds for disapproval, set against, oppose," literally "to put or throw before or against," frequentative of obicere (see object (n.)). Related: Objected; objecting.
objectification (n.): 1860, noun of action from objectify.
objectify (v.): 1838, from Medieval Latin objectum (see object (n.)) + -fy. Related: Objectified; objectifying.
objection (n.): late 14c., from Old French objeccion "reply, retort" (12c.) and directly from Late Latin obiectionem (nominative obiectio), "a throwing or putting before," noun of action from past participle stem of Latin obicere "to oppose" (see object (n.)).
objectionable (adj.): 1781, from objection + -able. Related: Objectionably.
objective (adj.): 1610s, originally in the philosophical sense of "considered in relation to its object" (opposite of subjective), formed on pattern of Medieval Latin objectivus, from objectum "object" (see object (n.)) + -ive. Meaning "impersonal, unbiased" is first found 1855, influenced by German objektiv. Related: Objectively.
objective (n.): 1738, "something objective to the mind," from objective (adj.). Meaning "goal, aim" (1881) is from military term objective point (1852), reflecting a sense evolution in French.
objectivism (n.): 1854 in philosophical sense, "the doctrine that knowledge is based on objective reality," from objective (adj.) + -ism.
objectivity (n.): 1803, from Medieval Latin objectivus, from Latin objectus (see object (n.)) + -ity.