one (n.) Look up one at Dictionary.com
c. 1200, from Old English an (adjective, pronoun, noun) "one," from Proto-Germanic *ainaz (cognates: Old Norse einn, Danish een, Old Frisian an, Dutch een, German ein, Gothic ains), from PIE *oi-no- "one, unique" (cognates: Greek oinos "ace (on dice);" Latin unus "one;" Old Persian aivam; Old Church Slavonic -inu, ino-; Lithuanian vienas; Old Irish oin; Breton un "one").

Originally pronounced as it still is in only, and in dialectal good 'un, young 'un, etc.; the now-standard pronunciation "wun" began c. 14c. in southwest and west England (Tyndale, a Gloucester man, spells it won in his Bible translation), and it began to be general 18c. Use as indefinite pronoun influenced by unrelated French on and Latin homo.

One and only "sweetheart" is from 1906. One of those things "unpredictable occurrence" is from 1934. Slang one-arm bandit "a type of slot machine" is recorded by 1938. One-night stand is 1880 in performance sense; 1963 in sexual sense. One of the boys "ordinary amiable fellow" is from 1893. One-track mind is from 1927. Drinking expression one for the road is from 1950 (as a song title).
one-horse (adj.) Look up one-horse at Dictionary.com
"small-scale, petty" 1853, American English, colloquial, in reference to towns so small they only had one horse.
one-liner (n.) Look up one-liner at Dictionary.com
"short joke, witty remark," 1969, from one + line.
one-of-a-kind Look up one-of-a-kind at Dictionary.com
adjectival phrase attested from 1961.
one-off (n.) Look up one-off at Dictionary.com
"single example of a manufactured product," 1934, from one + off. Later given figurative extension.
one-shot (adj.) Look up one-shot at Dictionary.com
1907, "achieved in a single attempt" (original reference is to golf), from one + shot (n.). Meaning "happening or of use only once" is from 1937.
one-sided (adj.) Look up one-sided at Dictionary.com
1833, "dealing with one side of a question or dispute," from one + side (n.). Related: One-sidedly; one-sidedness.
one-upsmanship (n.) Look up one-upsmanship at Dictionary.com
1952, from noun phrase one up "scoring one more point than one's opponent" (1919).
one-way (adj.) Look up one-way at Dictionary.com
1906, in reference to travel tickets; 1914 in reference to streets; 1940 in reference to windows, mirrors, etc.; from one + way (n.).
Oneida Look up Oneida at Dictionary.com
Iroquois tribe of upper N.Y. state, who later moved in part to Wisconsin, 1666, named for its principal settlement, from Oneida onenyote', literally "erected stone," containing -neny- "stone" and -ot- "to stand."
oneiric (adj.) Look up oneiric at Dictionary.com
1859, from Greek oneiros "a dream" (see oneiro-) + -ic.
oneiro- Look up oneiro- at Dictionary.com
before vowels oneir-, word-forming element meaning "dream," from Greek oneiros "a dream," from PIE *oner- "dream."
oneirocritic (n.) Look up oneirocritic at Dictionary.com
"a judge or interpreter of dreams," 1650s from Greek oneirokritikos "pertaining to the interpretation of dreams," from oneirokrites "interpreter of dreams," from oneiros "a dream" (see oneiro-) + krites "discerner, judge" (see critic).
oneirocritical (adj.) Look up oneirocritical at Dictionary.com
1580s, from oneiro- + critic + -al (1).
oneiromancy (n.) Look up oneiromancy at Dictionary.com
1650s; see oneiro- + -mancy. Greek had oneiromantis "an interpreter of dreams." Related: oneiromantic.
oneness (n.) Look up oneness at Dictionary.com
1590s, from one + -ness. A re-formation of Middle English onnesse, which vanished by 13c.
onerous (adj.) Look up onerous at Dictionary.com
late 14c., from Old French onereus, honereus (14c., Modern French onéreux) and directly from Latin onerosus, from onus (genitive oneris) "burden" (see onus).
oneself Look up oneself at Dictionary.com
1540s, one's self. Hyphenated 18c.; written as one word from c. 1827, on model of himself, itself, etc.
ongoing (adj.) Look up ongoing at Dictionary.com
also on-going, 1877, from on + going (see go).
onion (n.) Look up onion at Dictionary.com
early 12c., from Anglo-French union, Old French oignon "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), colloquial rustic Roman for "a kind of onion," also "pearl" (via notion of a string of onions), literally "one, unity;" sense connection is the successive layers of an onion, in contrast with garlic or cloves.

Old English had ynne (in ynne-leac), from the same Latin source, which also produced Irish inniun, Welsh wynwyn and similar words in Germanic. In Dutch, the ending in -n was mistaken for a plural inflection and new singular ui formed. The usual Indo-European name is represented by Greek kromion, Irish crem, Welsh craf, Old English hramsa, Lithuanian kremuse.

The usual Latin word was cepa, a loan from an unknown language; it is the source of Old French cive, Old English cipe, and, via Late Latin diminutive cepulla, Italian cipolla, Spanish cebolla, Polish cebula. German Zwiebel also is from this source, but altered by folk etymology in Old High German (zwibolla) from words for "two" and "ball." Onion ring is attested from 1952.

Onion dome attested from 1956; onion grass from 1883; onion skin as a type of paper from 1892. Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion "anvil."
oniony (adj.) Look up oniony at Dictionary.com
1838, from onion + -y (2). Related: Onioniness.
online (adj.) Look up online at Dictionary.com
in reference to computers, "directly connected to a peripheral device," 1950 (originally as on-line).
onlooker (n.) Look up onlooker at Dictionary.com
c. 1600, from on + agent noun from look (v.).
only (adj.) Look up only at Dictionary.com
Old English ænlic, anlic "only, unique, solitary," literally "one-like," from an "one" (see one) + -lic "-like" (see -ly (1)). Use as an adverb and conjunction developed in Middle English. Distinction of only and alone (now usually in reference to emotional states) is unusual; in many languages the same word serves for both. German also has a distinction in allein/einzig. Phrase only-begotten (mid-15c.) is biblical, translating Latin unigenitus, Greek monogenes. The Old English form was ancenned.
onnagata (n.) Look up onnagata at Dictionary.com
in Kabuki and similar drama, a man who plays female roles, 1901, from Japanese, from onna "woman" + kata "figure."
onomastic (adj.) Look up onomastic at Dictionary.com
1716, from French onomastique (17c.), from Greek onomastikos "of or belonging to naming," from onomastos "named," verbal adjective of onomazein "to name," from onoma "name" (see name).
onomastics (n.) Look up onomastics at Dictionary.com
"scientific study of names and naming," 1936, from onomastic; also see -ics.
onomatopoeia (n.) Look up onomatopoeia at Dictionary.com
1570s, from Late Latin onomatopoeia, from Greek onomatopoiia "the making of a name or word" (in imitation of a sound associated with the thing being named), from onomatopoios, from onoma (genitive onomatos) "word, name" (see name (n.)) + a derivative of poiein "compose, make" (see poet). Related: Onomatopoeic; onomatopoeial.
onomatopoeic (adj.) Look up onomatopoeic at Dictionary.com
1860, from French onomatopoéique or else from onomatopoeia + -ic.
Onondaga Look up Onondaga at Dictionary.com
tribe in the Iroquois Confederacy, 1684, named for its principal settlement, from Onondaga onontake, literally "on the hill."
onset (n.) Look up onset at Dictionary.com
1530s, "attack, assault," from on + set (n.); compare verbal phrase to set (something) on (someone). Weaker sense of "beginning, start" first recorded 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s.
onslaught (n.) Look up onslaught at Dictionary.com
1620s, anslaight, somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Spelling influenced by obsolete (since c. 1400) English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)). No record of its use in 18c.; apparently revived by Scott.
Ontario Look up Ontario at Dictionary.com
from Mohawk (Iroquoian) ontari:io "beautiful lake" or "great lake," from /-qtar-/ "lake, river." Related: Ontarian.
ontic (adj.) Look up ontic at Dictionary.com
1949, from onto- + -ic.
onto (prep.) Look up onto at Dictionary.com
1580s, as on to, from on + to. Appeared much later than parallel into. As a closed compound (on analogy of into), first recorded 1715.
onto- Look up onto- at Dictionary.com
word-forming element meaning "a being, individual; being, existence," from Greek onto-, from stem of on (genitive ontos) "being," neuter present participle of einai "to be" (see essence).
ontogeny (n.) Look up ontogeny at Dictionary.com
"development of an individual," 1872, from onto- + -geny. Related: Ontogenic; ontogenesis.
ontological (adj.) Look up ontological at Dictionary.com
1782, from ontology + -ical. Related: Ontologically
ontology (n.) Look up ontology at Dictionary.com
"metaphysical science or study of being," 1660s (Gideon Harvey), from Modern Latin ontologia (c. 1600), from onto- + -logy.
onus (n.) Look up onus at Dictionary.com
1640s, from Latin onus "load, burden," figuratively "tax, expense; trouble, difficulty," from PIE *en-es- "burden" (source of Sanskrit anah "cart, wagon"). Hence legal Latin onus probandi (1722), literally "burden of proving."
onward (adv.) Look up onward at Dictionary.com
late 14c., from on + -ward. The form onwards, with adverbial genitive -s-, is attested from c. 1600.
onwards (adv.) Look up onwards at Dictionary.com
see onward.
onymous (adj.) Look up onymous at Dictionary.com
1775, coined to provide an opposite to anonymous. Related: Onymously.
onyx (n.) Look up onyx at Dictionary.com
mid-13c., from Old French oniche "onyx" (12c.), and directly from Latin onyx (genitive onychis), from Greek onyx "onyx-stone," originally "claw, fingernail" (see nail (n.)). So called because the mineral's color sometimes resembles that of a fingernail, pink with white streaks.
oo- Look up oo- at Dictionary.com
word-forming element meaning "egg, eggs," from Greek oon "egg," cognate with Latin ovum, Old Norse egg (see egg (n.)).
oocyte (n.) Look up oocyte at Dictionary.com
1895, from oo- + -cyte (see cyto-).
oodles (n.) Look up oodles at Dictionary.com
"lots," 1869, American English (originally in a Texas context), perhaps from the caboodle in kit and caboodle (see kit).
oogenesis (n.) Look up oogenesis at Dictionary.com
"formation of the ovum," 1892, from oo- + -genesis "birth, origin, creation."
ooh Look up ooh at Dictionary.com
exclamation of pain, surprise, wonder, etc., 1916. Combined with aah from 1953. Ooh-la-la, exclamation of surprise or appreciation, is attested 1924, from French and suggestive of the supposed raciness of the French.
oolite (n.) Look up oolite at Dictionary.com
"rock consisting of fine grains of carbonate of lime," 1785, from Modern Latin oolites, from oo-, comb. form of Greek oon "egg" (cognate with Old English æg, see egg (n.)) + lithos "stone" (see litho-). So called because the rock resembles the roe of fish.