Online Etymology Dictionary

obsessed (adj.): mid-15c., "tormented, obsessed," past participle adjective from obsess. Originally especially "possessed" by a devil, etc.
obsession (n.): 1510s, "action of besieging," from French obsession and directly from Latin obsessionem (nominative obsessio) "siege, blockade, a blocking up," noun of action from past participle stem of obsidere "to besiege" (see obsess). Later (c. 1600), "hostile action of an evil spirit" (like possession but without the spirit actually inhabiting the body). Transferred sense of "action of anything which engrosses the mind" is from 1670s. Psychological sense is from 1901.
obsessive (adj.): 1911, from obsess + -ive. Related: Obsessively. Obsessive-compulsive is attested from 1927.
obsidian (n.): "dark, hard volcanic rock," 1650s, from Latin obsidianus, misprint of Obsianus (lapis) "(stone) of Obsius," name of a Roman alleged by Pliny to have found this rock in Ethiopia.
obsolesce (v.): 1801, from Latin obsolescere "to grow old, wear out, lose value, become obsolete," inchoative of obsolere "fall into disuse" (see obsolete). Related: Obsolesced; obsolescing.
obsolescence (n.): 1809; see obsolescent + -ence. Phrase Planned obsolescence coined 1932, revived as a disparaging term 1950s.
obsolescent (adj.): 1755, from Latin obsolescentum (nominative obsolescens), present participle of obsolescere "fall into disuse" (see obsolete).
obsolete (adj.): 1570s, from Latin obsoletus "grown old, worn-out," past participle of obsolescere "fall into disuse, be forgotten about, become tarnished," which probably is from ob "away" (see ob-) + an expanded form of solere "to be used to, be accustomed" (see insolent).
obstacle (n.): mid-14c., from Old French obstacle, ostacle "opposition, obstruction, hindrance" (13c.) or directly from Latin obstaculum "a hindrance, obstacle," with instrumental suffix *-tlom + obstare "stand before, stand opposite to, block, hinder, thwart," from ob "against" (see ob-) + stare "to stand," from PIE root *stā- "to stand" (see stet).
The lover thinks more often of reaching his mistress than the husband of guarding his wife; the prisoner thinks more often of escaping than the gaoler of shutting his door; and so, whatever the obstacles may be, the lover and the prisoner ought to succeed. [Stendhal, "Charterhouse of Parma"]
Obstacle course is attested from 1891.
obstetric (adj.): 1742, from Modern Latin obstetricus "pertaining to a midwife," from obstetrix (genitive obstetricis) "midwife," literally "one who stands opposite (the woman giving birth)," from obstare "stand opposite to" (see obstacle). The true adjective would be obstetricic, "but only pedantry would take exception to obstetric at this stage of its career." [Fowler]. Related: Obstetrical.
obstetrician (n.): 1828, from Latin obstetricia "midwifery," from obstetricus (see obstetric) on model of physician.
obstetrics (n.): "science of midwifery," 1819, from obstetric (adj.); also see -ics.
obstinacy (n.): late 14c., from Medieval Latin obstinatia, from obstinatus (see obstinate).
obstinance (n.): early 15c., from Medieval Latin obstinantia, from obstinantem, from obstinatus "resolved, determined, resolute" (see obstinate).
obstinate (adj.): mid-14c., from Latin obstinatus "resolute, resolved, determined, inflexible, stubborn," past participle of obstinare "persist, stand stubbornly, set one's mind on," from ob "by" (see ob-) + stinare (related to stare "stand") from PIE *ste-no-, from root *stā- "to stand" (see stet). Related: Obstinately.
obstipation (n.): 1590s, from Latin obstipationem (nominative obstipatio), noun of action from *obstipare "action of blocking or stopping up," from ob- (see ob-) + stipare "to press together, to pack" (see stiff (adj.)).
obstreperous (adj.): c. 1600, from Latin obstreperus "clamorous," from obstrepere "drown with noise, make a noise against, oppose noisily," from ob "against" (see ob-) + strepere "make a noise," from PIE *strep-, said to be imitative (compare Latin stertare "to snore," Old Norse þrapt "chattering," Old English þræft "quarrel"). Related: Obstreperously; obstreperousness.
obstruct (v.): 1610s, a back-formation from obstruction or else from Latin obstructus, past participle of obstruere "to block, to stop up" (see obstruction). Related: Obstructed; obstructing.
obstruction (n.): 1530s, from Latin obstructionem (nominative obstructio) "an obstruction, barrier, a building up," noun of action from past participle stem of obstruere "build up, block, block up, build against, stop, bar, hinder," from ob "against" (see ob-) + struere "to pile, build" (see structure (n.)).
obstructionism (n.): 1879, from obstruction + -ism.
obstructionist (n.): 1846, from obstruction + -ist.
obstructive (adj.): 1610s, from Latin obstruct-, past participle stem of obstruere (see obstruction) + -ive.
obtain (v.): early 15c., from Middle French obtenir "acquire, obtain" (14c.), from Latin obtinere "hold, hold fast, take hold of, get possession of, acquire," from ob "to" (though perhaps intensive in this case; see ob-) + tenere "to hold" (see tenet). Related: Obtained; obtaining.
obtainable (adj.): 1610s, from obtain + -able. Related: Obtainability.
obtrude (v.): 1550s, from Latin obtrudere "to thrust into, press upon," from ob "toward" (see ob-) + trudere "to thrust," "to thrust, push," from PIE *treud- "to press, push, squeeze" (see threat). Related: Obtruded; obtruding.
obtrusion (n.): 1570s, from Latin obtrusionem (nominative obtrusio), noun of action from past participle stem of obtrudere (see obtrude).
obtrusive (adj.): 1660s, from Latin obtrus-, past participle stem of obtrudere (see obtrude) + -ive. Related: Obtrusively; obtrusiveness.
obtund (v.): c. 1400, (transitive) "to render dead, make dull," used occasionally in English, especially in medical jargon; from Latin obtundere "to blunt, make dull, weaken, exhaust," literally "to beat against" (see obtuse). Related: Obtundation; obtunded.
obtuse (adj.): early 15c., "dull, blunted," from Middle French obtus (fem. obtuse), from Latin obtusus "blunted, dull," also used figuratively, past participle of obtundere "to beat against, make dull," from ob "against" (see ob-) + tundere "to beat," from PIE *(s)tud-e- "to beat, strike, push, thrust," from root *(s)teu- "to push, stick, knock, beat" (source also of Latin tudes "hammer," Sanskrit tudati "he thrusts"). Sense of "stupid" is first found c. 1500. Related: Obtusely; obtuseness.
obverse (adj.): "turned toward the observer, frontal," 1650s, from Latin obversus "turned against, directed toward," past participle of obvertere "to turn toward or against," from ob "toward" (see ob-) + vertere "to turn" (see versus). According to OED, not in common use until the end of the 18th century. The noun, in reference to coins, medals, etc. (opposite of reverse), is attested from 1650s. Related: Obversely.
obviate (v.): 1590s, "to meet and do away with," from Late Latin obviatus, past participle of obviare "act contrary to, go against," from Latin obvius "that is in the way, that moves against" (see obvious). Related: Obviated; obviating.
obviation (n.): early 15c., from Medieval Latin obviationem (nominative obviatio), noun of action from past participle stem of obviare (see obviate).
obvious (adj.): 1580s, "frequently met with," from Latin obvius "that is in the way, presenting itself readily, open, exposed, commonplace," from obviam (adv.) "in the way," from ob "against" (see ob-) + viam, accusative of via "way" (see via). Meaning "plain to see, evident" is first recorded 1630s. Related: Obviously; obviousness.
oc-: assimilated form of ob- before -c-.
ocarina (n.): 1877, from Italian ocarina, diminutive of oca "goose" (so called for its shape), from Vulgar Latin *auca, from Latin avicula "small bird," diminutive of avis "bird" (see aviary).
Occam's razor: when two competing hypotheses explain the data equally well, choose the simpler. Or, as Sir William Hamilton puts it, "Neither more, nor more onerous, causes are to be assumed, than are necessary to account for the phenomena." Named for English philosopher William of Ockham or Occam (c. 1285-c. 1349), who expressed it with Entia non sunt multiplicanda praeter ncccssitatem.
So called after William of Occam (died about 1349): but, as a historical fact, Occam does not make much use of this principle, which belongs rather to the contemporary nominalist William Durand de St. Pourçain (died 1332). [Century Dictionary]
occasion (n.): late 14c., "opportunity; grounds for action, state of affairs that makes something else possible; a happening, occurrence," from Old French ochaison, ocasion "cause, reason, excuse, pretext; opportunity" (13c.) or directly from Latin occasionem (nominative occasio) "opportunity, appropriate time," in Late Latin "cause," from occasum, occasus, past participle of occidere "fall down, go down," from ob "down, away" (see ob-) + cadere "to fall" (see case (n.1)). The notion is of a "falling together," or juncture, of circumstances.
occasion (v.): mid-15c., "to bring (something) about," from occasion (n.), or else from Old French occasionner "to cause," from Medieval Latin occasionare, from Latin occasionem (see occasion (n.)). Related: Occasioned; occasioning.
occasional (adj.): late 14c., "occurring now and then," from occasion (n.) + -al (1). Meaning "casual" is 1560s. Meaning "happening on or pertaining to a particular occasion" is from 1630s. Of furniture, etc., from 1749.
occasionally (adv.): c. 1400, "happening on some particular occasion," also "sometimes, happening as occasion presents itself, without regularity," from occasional + -ly (2).
Occident (n.): late 14c., "western part" (of the heavens or earth), from Old French occident (12c.) or directly from Latin occidentem (nominative occidens) "western sky, sunset, part of the sky in which the sun sets," noun use of adjective meaning "setting," from present participle of occidere "fall down, go down" (see occasion (n.)).
occidental (adj.): c. 1400, from Old French occidental (14c.) and directly from Latin occidentalis "western," from occidentem (see occident). As a capitalized noun meaning "a Western person" (opposed to Oriental) from 1857.
occipital (adj.): 1540s, from Middle French occipital, from Medieval Latin occipitalis, from Latin occiput (genitive occipitis) "back of the skull," from ob "against, behind" (see ob-) + caput "head" (see capitulum).
Occitan (n.): "Old or modern Provençal; langue d'Oc," 1940, also "the northern variant of modern Provençal;" from French oc (see Languedoc).
occlude (v.): 1590s, from Latin occludere (past participle occlusus) "shut up, close up," from ob "against, up" (see ob-) + claudere "to shut, close" (see close (v.)). Of teeth, 1888 (also see occlusion). Related: Occluded; occluding.
occlusion (n.): 1640s, from Medieval Latin occlusionem (nominative occlusio), noun of action from past participle stem of Latin occludere (see occlude). Dentistry sense is from 1880.
occlusive (adj.): 1867, from Latin occlus-, past participle stem of occludere (see occlude) + -ive.
occult (adj.): 1530s, "secret, not divulged," from Middle French occulte and directly from Latin occultus "hidden, concealed, secret," past participle of occulere "cover over, conceal," from ob "over" (see ob-) + a verb related to celare "to hide," from PIE root *kel- (2) "to cover, conceal" (see cell). Meaning "not apprehended by the mind, beyond the range of understanding" is from 1540s. The association with the supernatural sciences (magic, alchemy, astrology, etc.) dates from 1630s.
occultation (n.): early 15c., "disguise or concealment of identity," from Latin occultationem (nominative occultatio), noun of action from past participle stem of occultare "to hide, conceal," frequentative of occulere (see occult).
occultism (n.): 1870, from occult + -ism. Related: Occultist.