| opisthognathous | adj. | having retreating jaws or jawline | ||
| opopanax | n. | gum resin made from various plants, some used for medicine and others for perfume | ||
| opsimath | n. | one who begins to learn late in life | ||
| optative | adj. | of verb mood expressing wish or desire | ||
| orante | adj. | (also orans or orant) prayerful posture, in which one stands with hands held out to the side with the palms up, while the elbows are held close to the body | ||
| ordure | n. | excrement, dung | ||
| organoleptic | adj. | of or related to perception by sensory organs | ||
| orgeat | n. | syrup made from almond paste mixed with rosewater or orange water; similar syrup made from barley | ||
| orichalcum | n. | metal prized by Ancient Greeks and Romans, of debated composition (or even existence) | ||
| oriel | n. | bay window; protrusion from wall in which a bay window may be sited | architecture | |
| oriflamme | n. | banner of St. Denis, red with two or three points; ensign or standard serving as a rallying point in a struggle; crappy advertising banner used at conventions, exhibits, etc. | ||
| orison | n. | prayer | ||
| orlop | n. | fourth or lowest deck of a sailing ship | nautical | |
| ormolu | n. | gold-colored alloy of copper and zinc | ||
| orography | n. | scientific study of mountains; mountain features of a region | ||
| oroide | n. | gold-colored alloy of tin or zinc with copper | ||
| orotund | adj. | (of speech or voice) full and clear; (of style) pompous, inflated | ||
| orphrey | n. | embroidered ornamental band or border on medieval vestments | ||
| orpiment | n. | yellow arsenic sulfide used as pigment | ||
| orrery | n. | clockwork mechanism showing planetary motion | ||
| ort | n. | leftover | ||
| orthoepy | n. | study of pronunciation; correct pronunciation | ||
| orthognathous | adj. | having a jaw nearly in line with the forehead and so neither jutting nor receding | ||
| orthogonal | adj. | perpendicular to, at right angles; of functions whose product has an integral equal to zero; irrelevant | ||
| orthopnea | n. | difficulty breathing except in standing or sitting upright position | ||
| oscitant | adj. | yawning, gaping; tired, drowsy | ||
| osetra | n. | caviar from the eponymous Russian sturgeon | ||
| osier | n. | willow twigs much used in basket-work | ||
| osoji | n. | massive cleaning before the new year | ||
| ostension | n. | demonstration, presentation; showing of the sacrament for adoration | ||
| ostentatious | adj. | intended to attract notice | ||
| ostinato | n. | musical phrase or rhythm which is repeated over and over again | ||
| ostler | n. | person responsible for horses, esp. at an inn | ||
| ostracon | n. | (also ostrakon) inscribed potsherd, esp. that used as vote for ostracism | ||
| otaku | n. | obsessive fanboy, esp. of computers, anime, or manga | ||
| otic | adj. | of the ear | ||
| otiose | adj. | superfluous, useless; nugatory | ||
| ouch | n. | brooch, clasp, buckle; setting for precious stone | ||
| ounce | n. | snow lepoard; lynx or cougar | ||
| ousel | n. | Eurasian blackbird; American dipper | ||
| out at elbows | adj. | poorly dressed, shabby; needy, poor | ||
| out of spoons | adj. | exhausted, lacking energy (based on so-called Spoon Theory positing that people (especially those with certain disabilities) have finite quanta of energy to accomplish mental or physical tasks) | slang | |
| outrance | n. | furthest extremity, at the very limits (of propriety) | obsolete | |
| outwith | prep. | outside | ||
| ouzel | n. | aquatic bird found in the Rocky Mountains | ||
| overlearn | v. | to learn or memorize more than needed for existing use | ||
| overmantel | n. | panel or decoration above a mantel | ||
| overmorrow | adv. | on the day after tomorrow | archaic | |
| overparticular | adj. | fussy, very fastidious | ||
| overprocrastination | n. | overindulgence in putting things off |
