| oppressive | (adjective) marked by unjust severity or arbitrary behavior | Synonyms: tyrannical, tyrannous |
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| (adjective) weighing heavily on the senses or spirit | - |
| oppressively | (adverb) in a heavy and oppressive way | - |
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| oppressiveness | (noun) unwelcome burdensome difficulty | Synonyms: burdensomeness, heaviness, onerousness |
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| (noun) a feeling of being oppressed | Synonyms: oppression |
| oppressor | (noun) a person of authority who subjects others to undue pressures | - |
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| opprobrious | (adjective) (used of conduct or character) deserving or bringing disgrace or shame | Synonyms: black, disgraceful, ignominious, inglorious, shameful |
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| (adjective) expressing offensive reproach | Synonyms: abusive, scurrilous |
| opprobrium | (noun) a state of extreme dishonor | Synonyms: infamy |
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| (noun) state of disgrace resulting from public abuse | Synonyms: obloquy |
| oppugn | (verb) challenge the accuracy, probity, or propriety of | Synonyms: call into question, question |
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| opsin | (noun) retinal protein formed by the action of light on rhodopsin | - |
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| opsonin | (noun) an antibody in blood serum that attaches to invading microorganisms and other antigens to make them more susceptible to the action of phagocytes | - |
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| opsonisation | (noun) process whereby opsonins make an invading microorganism more susceptible to phagocytosis | Synonyms: opsonization |
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| opsonization | (noun) process whereby opsonins make an invading microorganism more susceptible to phagocytosis | Synonyms: opsonisation |
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| opsonize | (verb) make (cells) more susceptible to the action of phagocytes | - |
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| opt | (verb) select as an alternative over another | Synonyms: choose, prefer |
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| optative | (adjective) relating to a mood of verbs in some languages | - |
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| (adjective) indicating an option or wish | - |
| (noun) a mood (as in Greek or Sanskrit) that expresses a wish or hope; expressed in English by modal verbs | Synonyms: optative mood |
| optic | (adjective) of or relating to or resembling the eye | Synonyms: ocular, ophthalmic, optical |
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| (adjective) relating to or using sight | Synonyms: ocular, optical, visual |
| (noun) the organ of sight | Synonyms: eye, oculus |
| optical | (adjective) of or relating to or resembling the eye | Synonyms: ocular, ophthalmic, optic |
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| (adjective) of or relating to or involving light or optics | - |
| (adjective) relating to or using sight | Synonyms: ocular, optic, visual |
| optically | (adverb) in an optical manner | - |
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| optician | (noun) a worker who makes glasses for remedying defects of vision | Synonyms: lens maker |
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| optics | (noun) optical properties | - |
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| (noun) the branch of physics that studies the physical properties of light | - |
| optimal | (adjective) most desirable possible under a restriction expressed or implied | Synonyms: optimum |
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| optimally | (adverb) in an optimal and most desirable way | - |
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| optimisation | (noun) the act of rendering optimal | Synonyms: optimization |
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| optimise | (verb) act as an optimist and take a sunny view of the world | Synonyms: optimize |
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| (verb) make optimal; get the most out of; use best | Synonyms: optimize |
| (verb) modify to achieve maximum efficiency in storage capacity or time or cost | Synonyms: optimize |
| optimism | (noun) a general disposition to expect the best in all things | - |
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| (noun) the optimistic feeling that all is going to turn out well | - |
| optimist | (noun) a person disposed to take a favorable view of things | - |
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| optimistic | (adjective) expecting the best in this best of all possible worlds | - |
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| (adjective) expecting the best | Synonyms: affirmative |
| optimistically | (adverb) with optimism; in an optimistic manner | - |
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| optimization | (noun) the act of rendering optimal | Synonyms: optimisation |
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| optimize | (verb) act as an optimist and take a sunny view of the world | Synonyms: optimise |
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| (verb) make optimal; get the most out of; use best | Synonyms: optimise |
| (verb) modify to achieve maximum efficiency in storage capacity or time or cost | Synonyms: optimise |
| optimum | (adjective) most desirable possible under a restriction expressed or implied | Synonyms: optimal |
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| (noun) most favorable conditions or greatest degree or amount possible under given circumstances | - |
| option | (noun) the act of choosing or selecting | Synonyms: choice, pick, selection |
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| (noun) one of a number of things from which only one can be chosen | Synonyms: alternative, choice |
| (noun) the right to buy or sell property at an agreed price; the right is purchased and if it is not exercised by a stated date the money is forfeited | - |
| optional | (adjective) possible but not necessary; left to personal choice | - |
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| optionality | (noun) Quality or state in which choice or discretion is allowed. | Synonyms: electiveness |
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| optionally | (adverb) in an optional manner | - |
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| optometrist | (noun) a person skilled in testing for defects of vision in order to prescribe corrective glasses | Synonyms: oculist |
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| optometry | (noun) the practice of an optometrist | - |
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| opulence | (noun) wealth as evidenced by sumptuous living | Synonyms: luxuriousness, luxury, sumptuousness |
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| opulent | (adjective) ostentatiously rich and superior in quality | Synonyms: deluxe, gilded, grand, lush, luxurious, princely, sumptuous |
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| opulently | (adverb) in a sumptuous and opulent manner | Synonyms: sumptuously |
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| opus | (noun) a musical work that has been created | Synonyms: composition, musical composition, piece, piece of music |
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