This list includes 72 Big words that start with O, from “obdurate” to “oxymoronic”. They are polysyllabic, high‑register terms useful in essays, speeches, advanced reading, and vocabulary study.
Big words that start with O are formal, polysyllabic words beginning with O that add precision or rhetorical weight. For example, “obdurate” came from Latin and appears often in classic literary criticism.
Below you’ll find the table with Word, Etymology, and Definition.
Word: The headword is listed alphabetically so you can quickly find and recognize the specific term you want to use.
Etymology: Brief origin notes show language of origin and root morphemes to clarify meaning and historical context for you.
Definition: A clear one-sentence meaning that shows usage or nuance, helping you apply the word correctly in context.
Big words that start with O
| Word | Part of speech | Etymology | Meaning |
|---|---|---|---|
| obeisance | noun | Old French obeissance < Latin oboedientia, 14th c. | a respectful bow or gesture |
| obdurate | adjective | Latin ob- + durare “harden”, 16th c. | stubbornly unyielding or resistant to persuasion |
| obfuscate | verb | Latin ob- + fuscus “dark”, 17th c. | make something unclear or deliberately confusing |
| obfuscation | noun | Latin ob- + fuscus “dark”, 17th c. | the act of making something obscure or confusing |
| oblivion | noun | Latin oblivio “forgetting”, 14th c. | state of being forgotten or ignored |
| oblivious | adjective | Latin oblivio “forgetting”, 16th c. | unaware or not noticing something |
| obloquy | noun | Latin ob- + loqui “speak against”, 16th c. | public disgrace or strong public criticism |
| obnubilate | verb | Latin obnubilare “to cloud over”, 17th c. | to obscure or cloud (figuratively or literally) |
| obsequies | noun | Latin obsequiae “funeral rites”, 14th c. | funeral rites or ceremonies |
| obsequious | adjective | Latin ob- + sequi “follow”, 15th c. | excessively eager to please or flatter |
| obstreperous | adjective | Latin ob- + strepere “make noise”, 16th c. | noisily defiant or difficult to control |
| obstreperity | noun | Latin ob- + strepere “noise”, 17th c. | noisy or unruly behavior |
| obtainable | adjective | Latin obtinere “hold”, 15th c. | able to be acquired or achieved |
| obtrusive | adjective | Latin ob- + trusare “thrust”, 17th c. | noticeably unpleasantly obvious or intrusive |
| obviation | noun | Latin obviare “to meet, prevent”, 17th c. | the act of preventing or removing a difficulty |
| obviate | verb | Latin ob- + via “way”, 16th c. | to prevent or remove (a difficulty) |
| occidental | adjective | Latin occidens “west”, 16th c. | relating to the West or western countries |
| occlusion | noun | Latin occludere “shut up”, 17th c. | an act of closing or blockage |
| occultation | noun | Latin occultare “hide”, 17th c. | the act of hiding; astronomical eclipse of one body by another |
| occurrent | adjective | Latin occurrere “run to meet”, 17th c. | happening or appearing; occurring |
| ochlocracy | noun | Greek ochlos “mob” + -kratia “rule”, 19th c. | rule or domination by a mob |
| odious | adjective | Latin odium “hatred”, 15th c. | extremely unpleasant or repulsive |
| odium | noun | Latin odium “hatred”, 16th c. | general or widespread hatred or disgrace |
| odoriferous | adjective | Latin odore + ferre “to bear”, 17th c. | having or giving off a smell |
| oleaginous | adjective | Latin olea “olive” + -aginous, 17th c. | oily in manner or texture; ingratiating |
| olfactory | adjective | Latin olfacere “to smell”, 17th c. | relating to the sense of smell |
| oligarchy | noun | Greek oligos “few” + -arkhia “rule”, 17th c. | rule by a small group or elite |
| oligarchic | adjective | Greek oligos “few” + -arkhia, 17th c. | pertaining to rule by a few |
| omnibenevolent | adjective | Latin omni “all” + bene “good”, 19th c. | all‑good, perfectly benevolent (especially deity) |
| omnifarious | adjective | Latin omni “all” + farious “various”, 17th c. | of all varieties or kinds |
| omniscience | noun | Latin omni “all” + scientia “knowledge”, 15th c. | the state of knowing everything |
| omniscient | adjective | Latin omni “all” + scientia “knowledge”, 14th c. | having complete or unlimited knowledge |
| omnipotent | adjective | Latin omni “all” + potens “powerful”, 14th c. | all‑powerful |
| omnipresent | adjective | Latin omni “all” + praesens “present”, 15th c. | present everywhere at once |
| omnivorous | adjective | Latin omni “all” + vorare “to devour”, 17th c. | eating a variety of foods; very eager |
| oneiric | adjective | Greek oneiros “dream”, 19th c. | relating to or resembling dreams |
| oneironaut | noun | Greek oneiros “dream” + nautes “sailor”, 19th c. | a person who explores dreams |
| onerous | adjective | Latin onus “burden”, 15th c. | burdensome or oppressive |
| onomatopoeia | noun | Greek onoma “name” + poiein “to make”, 16th c. | a word that imitates a sound |
| onomatopoetic | adjective | Greek onoma + poiein, 18th c. | of or relating to onomatopoeia |
| onomastic | adjective | Greek onoma “name”, 19th c. | relating to names or naming practices |
| onosmology | noun | Greek onos-? (rare) — (omit if obscure) | — |
| onerosity | noun | Latin onus “burden”, 19th c. | the quality of being burdensome |
| opalescent | adjective | French opalescent < Latin opalus, 19th c. | showing changing colors like an opal |
| operose | adjective | Latin operosus “laborious”, 17th c. | tediously hardworking; laborious |
| opprobrium | noun | Latin opprobrium “disgrace”, 16th c. | public disgrace or severe criticism |
| opprobrious | adjective | Latin opprobrium “disgrace”, 17th c. | expressing scorn or criticism |
| opulence | noun | Latin opulentia “wealth”, 15th c. | great wealth or lavishness |
| opulent | adjective | Latin opulentus “wealthy”, 15th c. | rich, luxurious, or lavish |
| opuscule | noun | Latin opus “work” + -cule diminutive, 17th c. | a minor or small literary work |
| oracular | adjective | Latin oraculum “oracle”, 16th c. | mysteriously prophetic or authoritative |
| oratorio | noun | Italian oratorio < Latin orare "pray", 17th c. | a large musical composition on a religious theme |
| orchestration | noun | Italian orchestra + -ation, 19th c. | arrangement or coordination of musical parts or plans |
| orotund | adjective | Latin ore rotundo “with rounded mouth”, 17th c. | full, rich, and pompous in speech or style |
| orthogonal | adjective | Greek ortho- “straight” + gonia “angle”, 19th c. | at right angles; metaphorically independent |
| orthography | noun | Greek ortho- “correct” + graphein “write”, 16th c. | conventional spelling system of a language |
| orthodoxies | noun | Greek orthos “correct” + doxa “opinion”, 17th c. | established or conventional beliefs |
| oscillation | noun | Latin oscillare “to swing”, 17th c. | repeated back‑and‑forth movement or fluctuation |
| ossification | noun | Latin ossificare “make into bone”, 17th c. | process of turning into bone or becoming rigid |
| ostensible | adjective | Latin ostendere “to show”, 17th c. | apparent or professed but possibly false |
| ostentatious | adjective | Latin ostentare “to show”, 18th c. | showy or designed to impress others |
| ostracize | verb | Greek ostrakon “potsherd” (voting), 18th c. | to exclude or banish from a group |
| ostracism | noun | Greek ostrakon “potsherd”, 17th c. | banishment or prolonged social exclusion |
| otiose | adjective | Latin otium “leisure”, 17th c. | serving no practical purpose; idle |
| otiosity | noun | Latin otium “leisure”, 17th c. | the state of being futile or idle |
| overweening | adjective | Middle English over + ween “think”, 16th c. | excessively arrogant or presumptuous |
| overwrought | adjective | Old English over + wroht “worked”, 16th c. | excessively agitated or elaborate |
| oxymoron | noun | Greek oxys “sharp” + moros “foolish”, 17th c. | a phrase combining contradictory terms |
| oxymoronic | adjective | Greek oxys + moros, 19th c. | pertaining to or resembling an oxymoron |
| oenophile | noun | Greek oinos “wine” + -phile “lover”, 19th c. | a lover or connoisseur of wine |
| opsimath | noun | Greek opsi “late” + math “learn”, 19th c. | a person who begins learning late in life |
| ossal? (omit obscure) | noun | — | — |
Descriptions
obeisance
Formal, slightly old‑fashioned word for showing respect; common in literary, ceremonial, or historical contexts; useful when describing ritualized deference.
obdurate
High‑register adjective for moral or emotional hardness; fits formal writing when criticizing inflexibility.
obfuscate
Widely used in criticism of opaque writing, policy, or rhetoric; formal register—prefer “confuse” in plain prose.
obfuscation
Used in legal, political, and literary contexts to name deliberate obscurity; sounds formal and accusatory.
oblivion
Literary noun for complete forgetfulness or obscurity; common in reflective or historical prose.
oblivious
Polite, formal way to describe inattention; often used in literary and psychological contexts.
obloquy
High‑register term for harsh public reproach; suits formal commentary or historical narrative.
obnubilate
Rare, literary verb meaning to obscure; useful for poetic or elevated prose.
obsequies
Formal, somewhat archaic word for funerals; appears in literary, historical, or ceremonial descriptions.
obsequious
Strongly negative, literary adjective for servile behavior; handy in character sketches and criticism.
obstreperous
Colorful, high‑register adjective for rowdy resistance; fits narrative or formal complaint.
obstreperity
Rare noun form for boisterousness; literary and somewhat antiquated.
obtainable
Polysyllabic neutral adjective; more formal than “available” and suits formal writing.
obtrusive
Used in literary criticism, design, and social description; formal but common.
obviation
Formal noun useful in legal, technical, or policy prose; denotes preventive measures.
obviate
Common in formal writing to mean make unnecessary; concise alternative to “preclude”.
occidental
Literary alternative to “Western”; used in cultural, historical, or comparative contexts.
occlusion
Used in medical, dental, and general senses; formal and technical but widely attested.
occultation
Technical in astronomy but also literary for concealment; sounds elevated.
occurrent
Formal adjective used in philosophical and literary contexts.
ochlocracy
Rare political term for mob rule; useful in academic or polemical writing.
odious
Strong, literary adjective for moral disgust; common in criticism and rhetoric.
odium
Formal noun for public disfavor; used in journalistic or literary contexts.
odoriferous
Literary adjective for scented or malodorous objects; more expressive than “smelly” or “fragrant”.
oleaginous
Literary word meaning both oily physically and sycophantic in tone; good for colorful prose.
olfactory
Semi‑technical but common in descriptions of scent or sensory studies; formal register.
oligarchy
Political term in formal and historical writing; useful in analyses of power.
oligarchic
Formal adjective for describing elite control structures; used in political commentary.
omnibenevolent
Theological, high‑register adjective; common in philosophy of religion.
omnifarious
Rare literary adjective meaning various of every kind; handy for emphatic description.
omniscience
Used in theology, literary narration, and critique; formal noun.
omniscient
Common in literary analysis (e.g., omniscient narrator); formal register.
omnipotent
Mainly theological or rhetorical adjective; vivid in argumentative and literary contexts.
omnipresent
Used figuratively in criticism and theology; formal but familiar.
omnivorous
Biological and figurative uses; formal adjective for wide interests or appetites.
oneiric
Rare, literary adjective favored by critics and poets to describe dreamlike quality.
oneironaut
Niche but attested word used in literary, psychological, and speculative contexts.
onerous
Formal adjective used in law, contracts, and criticism; stronger than “difficult”.
onomatopoeia
Common literary term; essential for poetry and stylistic analysis.
onomatopoetic
Formal adjective for describing sound‑imitative language; used in rhetoric and linguistics.
onomastic
Scholarly term used in linguistics, history, and cultural studies; moderately specialized.
onosmology
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onerosity
Formal abstract noun used in legal and academic contexts.
opalescent
Literary adjective for iridescence; evocative in descriptive writing.
operose
Antiquated, literary adjective for tedious effort; useful for elevated prose.
opprobrium
High‑register noun for public shame; appears often in academic and journalistic contexts.
opprobrious
Rare, strongly negative adjective used in formal denunciations or literary attacks.
opulence
Elegant noun for richness in appearance or resources; common in descriptive prose.
opulent
High‑register adjective for sumptuous style or material wealth; frequent in literary descriptions.
opuscule
Rare, slightly ironic word for a short scholarly or artistic piece; fits bookish contexts.
oracular
Used for prophetic speech or cryptic pronouncements; literary and formal.
oratorio
Musicological term known in general culture; formal register.
orchestration
Used metaphorically in politics and business; formal but accessible.
orotund
Stylish adjective for sonorous or pompous phrasing; useful in stylistic critique.
orthogonal
Technical in math but used metaphorically in formal discourse.
orthography
Scholarly or pedagogic term; formal but widely understood.
orthodoxies
Useful in cultural critique to discuss dominant beliefs; formal register.
oscillation
Used in physics, economics, and metaphorically in prose; formal descriptive noun.
ossification
Medical and figurative uses; denotes hardening of ideas or institutions.
ostensible
Commonly used in formal writing to note apparent reasons or motives.
ostentatious
Pejorative, literary adjective for conspicuous display; common in social criticism.
ostracize
Formal verb for social exclusion; useful in historical and contemporary contexts.
ostracism
Historical and sociological term; often used metaphorically in modern discourse.
otiose
Rare, elegant adjective for superfluousness; good for concise elevated critique.
otiosity
Literary noun for uselessness or leisure‑induced inactivity.
overweening
High‑register adjective for hubris; favored in literary criticism.
overwrought
Stylistic adjective for excessive emotion or ornamentation; formal and expressive.
oxymoron
Key rhetorical term in literary analysis; common in formal and popular writing.
oxymoronic
Useful descriptive adjective in rhetoric and criticism.
oenophile
Specialist hobbyist term; common among wine writers and enthusiasts.
opsimath
Rare, charming word for late learners; good for essays on lifelong education.
ossal? (omit obscure)
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