Etymology
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omo- 

before vowels om-, word-forming element meaning "raw, unripe," from Greek ōmos "raw," from PIE root *hemo- "raw, sharp-tasting" (source also of Sanskrit amah "raw, uncooked, unripe," Armenian hum; Old Irish om, Welsh of).

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-on 
subatomic particle suffix, from ion.
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onco- 

word-forming element meaning "bulk, mass," used from 19c., especially in medical use, "tumor," from Latinized form of Greek ogkos (pronounced "onkos") "bulk, size, mass, weight, swelling," in Modern Greek "tumor," a word of uncertain origin.

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-one 
chemical suffix, from Greek -one, female patronymic (as in anemone, "daughter of the wind," from anemos); in chemical use denoting a "weaker" derivative. Its use in forming acetone (1830s) gave rise to the specialized chemical sense.
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-oon 

spelling conventional in 15c.-17c. English to add emphasis to borrowed French nouns ending in stressed -on; also used to represent Italian -one, Spanish -ón; all from Latin -onem. Compare shalloon (1670s) for French chalon, a kind of material used for linings. The ending is used occasionally to form words in English, such as spittoon, quadroon, and some older ones no longer with us, such as shabberoon "disreputable person" (c. 1700).

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ophio- 

before vowels ophi-, word-forming element meaning "a snake, serpent," from Greek ophio-, combining form of ophis "serpent, a snake," probably from PIE *egwhi- (source also of Sanskrit ahi-, Avestan aži- "snake," and perhaps the first element of a Germanic word for "lizard," i.e. Old High German egi-dehsa). Hence ophiolatry "serpent-worship" (1862), and the 2c. sect of the Ophitæ, who revered the serpent as the symbol of divine wisdom. 

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ophthalmo- 

before vowels ophthalm-, word-forming element meaning "eye," mostly in plural, "the eyes," from Greek ophthalmos "eye," originally "the seeing," a word of uncertain origin. Perhaps from ōps "eye" (from PIE root *okw- "to see") + a form related to thalamos "inner room, chamber" (see thalamus), giving the whole a sense of "eye and eye socket," but Beekes rejects all this and finds it to be probably Pre-Greek.

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-or 

word-forming element making nouns of quality, state, or condition, from Middle English -our, from Old French -our (Modern French -eur), from Latin -orem (nominative -or), a suffix added to past participle verbal stems. Also in some cases from Latin -atorem (nominative -ator).

In U.S., via Noah Webster, -or is nearly universal (but not in glamour), while in Britain -our is used in most cases (but with many exceptions: author, error, tenor, senator, ancestor, horror etc.). The -our form predominated after c. 1300, but Mencken reports that the first three folios of Shakespeare's plays used both spellings indiscriminately and with equal frequency; only in the Fourth Folio of 1685 does -our become consistent.

A partial revival of -or on the Latin model took place from 16c. (governour began to lose its -u- 16c. and it was gone by 19c.), and also among phonetic spellers in both England and America (John Wesley wrote that -or was "a fashionable impropriety" in England in 1791).

Webster criticized the habit of deleting -u- in -our words in his first speller ("A Grammatical Institute of the English Language," commonly called the Blue-Black Speller) in 1783. His own deletion of the -u- began with the revision of 1804, and was enshrined in the influential "Comprehensive Dictionary of the English Language" (1806), which also established in the U.S. -ic for British -ick and -er for -re, along with many other attempts at reformed spelling which never caught on (such as masheen for machine). His attempt to justify them on the grounds of etymology and the custom of great writers does not hold up.

Fowler notes the British drop the -u- when forming adjectives ending in -orous (humorous) and derivatives in -ation and -ize, in which cases the Latin origin is respected (such as vaporize). When the Americans began to consistently spell it one way, however, the British reflexively hardened their insistence on the other. "The American abolition of -our in such words as honour and favour has probably retarded rather than quickened English progress in the same direction." [Fowler]

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ornitho- 

before vowels ornith-, word-forming element meaning "bird, birds," from combining form of Greek ornis (genitive ornithos; plural ornithēs) "a bird," in Attic generally "domestic fowl, cock or hen," which often was added to the specific name of the type of bird, from PIE *or- "large bird" (see erne).

For "bird" Greek also had ptēnon (plural peteina), related to pteron "wing," from the root meaning "to fly."

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-ory 
adjective and noun suffix, "having to do with, characterized by, tending to, place for," from Middle English -orie, from Old North French -ory, -orie (Old French -oir, -oire), from Latin -orius, -oria, -orium.

Latin adjectives in -orius, according to "An Etymological Dictionary of the French Language," tended to "indicate a quality proper to the action accomplished by the agent; as oratorius from orator; laudatorius from laudator. The neuter of these adjectives was early employed as a substantive, and usually denoted the place of residence of the agent or the instrument that he uses; as praetorium from praetor; dormitorium from dormitor; auditorium, dolatorium.

"These newer words, already frequent under the Empire, became exceedingly numerous at a later time, especially in ecclesiastical and scholastic Latin; as purgatorium, refectorium, laboratorium, observatorium, &c." [transl. G.W. Kitchin, Oxford, 1878]
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