| oblong | adj. | neither square nor circular, having length in one dimension greater than in another; elongated | ||
| obnoxious | adj. | exposed to danger | archaic | |
| obnubilation | n. | overclouding, darkening as with clouds; esp. clouding of mental facilities | ||
| obreption | n. | acquirement of a thing through fraud; (law) obtaining (a gift, a dispensation, etc.) by false statement | ||
| obsequy | n. | funeral rite | ||
| obstreperous | adj. | unruly; characterized by great noise or outcry | ||
| obstupefy | v. | to stupefy, esp. mentally | ||
| obtrude | v. | to thrust forward unduly; to push (something) upon a person, esp. without invitation | ||
| obtrusive | adj. | projecting or imposing in the way; overly forward | ||
| obtund | v. | to dull, to blunt | ||
| obus | n. | artillery shell | ||
| ochlocracy | n. | mob-rule | ||
| odium | n. | intense hatred or disgust; opprobrium, reproach | ||
| ods bodikins | int. | interjection palliating stronger phrase “God’s bodkins” (meaning the nails used to affix Jesus to the cross) | archaic | |
| oeillade | n. | amorous look or stare | ||
| of even date | adj. | of the same date | law | |
| ofay | n. | white person | slang | |
| office | n. | tip-off, warning | slang | |
| ogdoad | n. | group of eight, octet | ||
| ogee | n. | double curve with S-shaped cross-section | architecture | |
| ogive | n. | [statistics] graph of cumulative distribution function; [architecture] roundly pointed arch, rib supporting such an arch | ||
| oho | int. | exclamation of surprise, elation, recognition | ||
| oik | n. | oaf; lower class person | slang | |
| okta | n. | measurement of cloud cover equal to one eighth of the entire sky | ||
| oleograph | n. | picture printed in oil colors on cloth or canvas in imitation of an oil painting | ||
| oliguria | n. | scanty production of urine | ||
| omadhaun | n. | foolish male | ||
| omentum | n. | fold of peritoneum connecting stomach with spleen, liver, colon; caul | biology | |
| omnibus | n. | bus | archaic | |
| omnigenous | adj. | of all kinds | ||
| on foot of | adv. | because of | idiom | |
| on one’s uppers | adj. | very poor, destitute [from idea of poverty so acute that one’s shoes have worn away all the leather, so that only the upper portion remains] | British | |
| on the fiddle | adj. | of someone engaged in petty cheating | slang | |
| on the lash | adj. | drinking socially, usu. to excess | British | |
| on the tapis | adj. | under consideration, up for discussion | idiom | |
| onager | n. | Asiatic wild ass; ancient rock-hurling siege weapon | ||
| one-arm joint | n. | restaurant in urban centers with only single chairs with small table surface for diners’ use | slang | |
| oneiric | adj. | of or related to dreams or dreaming | ||
| oneirophrenia | n. | hallucinatory state of dreamlike perception, often caused by severe sleep deprivation | ||
| onomastic | adj. | of or related to personal or geographical names | ||
| ontic | adj. | noumenal, of the real rather than phenomenal | ||
| ontological | adj. | of or related to the nature of being or existence as such, about ontology; descriptive of category relations between concepts in a given field | ||
| oofy | adj. | loaded with money, rich, wealthy | British | |
| oolite | n. | spherical sedimentary rock formed in concentric layers | ||
| oology | n. | the study of eggs | ||
| operculum | n. | gill cover of a fish | zoology | |
| ophidian | adj. | of or related to snakes | ||
| ophiophagous | adj. | eating snakes | ||
| ophiophilist | n. | snake lover, ophiophile | ||
| ophthalmia | n. | eye inflammation, esp. conjunctivitis |
