Etymology
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os- 
frequent form of ob- before -c- and -t- in words from Latin.
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Oslo 

Norwegian capital city, a name probably based on Old Norse os "estuary, river mouth," in reference to the place's situation.

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sopor (n.)
Latin, "deep sleep, lethargy," from PIE *swep-os-, suffixed form of root *swep- "to sleep."
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usher (n.)
late 13c., "servant who has charge of doors and admits people to a chamber, hall, etc.," from Anglo-French usser (12c.), Old French ussier, uissier "porter, doorman," from Vulgar Latin *ustiarius "doorkeeper," variant of Latin ostiarius "door-keeper," from ostium "door, entrance," from os "mouth," from PIE *os- "mouth" (see oral). Fem. form usherette is attested from 1913, American English.
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Osmond 
masc. proper name, from Old English Osmund, literally "divine protection," from os "a god" (see Oscar) + -mund (see mount (n.1)).
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ilium (n.)
pelvic bone, 1706, Modern Latin, from Latin ilia (plural) "groin, flank, side of the body from the hips to the groin" (see ileum). In Middle English it meant "lower part of the small intestine." Vesalius gave the name os ilium to the "bone of the flank."
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ossicle (n.)

"a small bone; small, hard, bone-like nodule," 1570s, from Latin ossiculum, diminutive of os "bone" (from PIE root *ost- "bone").

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oral (adj.)

1620s, "uttered by the mouth or in words;" 1650s, "of or pertaining to the mouth," from Late Latin oralis, from Latin os (genitive oris) "mouth, opening, face, entrance," from PIE *os- "mouth" (source also of Sanskrit asan "mouth," asyam "mouth, opening," Avestan ah-, Hittite aish, Middle Irish a "mouth," Old Norse oss "mouth of a river," Old English or "beginning, origin, front").

Os was the usual word for "mouth" in Latin, but as the vowel distinction was lost it became similar in sound to os "bone" (see osseous). Thus bucca, originally "cheek" but used colloquially as "mouth," became the usual word for "mouth" (see bouche).

The psychological meaning "of the mouth as the focus of infantile sexual energy" (as in oral fixation) is attested from 1910. The sex-act sense is first recorded 1948, in Kinsey. As a noun, "oral examination," attested from 1876. Related: Orally (c. 1600); orality. 

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calcaneus (n.)
"heel-bone," 1751, from Latin (os) calcaneum "bone of the heel," from calcem (nominative calx (1)) "heel," a word of uncertain origin, perhaps from Etruscan. De Vaan lists as possible cognates Old Prussian culczi "hip," Lithuanian kulkšnis "ankle-(bone)," Bulgarian kalka "hip, thigh." Related: Calcaneal.
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oscitant (adj.)

"sleepy, drowsy, sluggish," literally "yawning, gaping," 1620s, from Latin oscitans "listless, sluggish, lazy," present participle of oscitare "to gape, yawn," from os citare "to move the mouth" (see oral and cite). Related: Oscitancy.

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