This list includes 82 Adjectives that start with O, from “oafish” to “oxidized”. They cover common, descriptive, and stylistic words useful for writing, editing, ESL practice, and vocabulary building.
Adjectives that start with O are single-word descriptors beginning with the letter O, used to modify nouns or pronouns. Many come from Latin or Old French, and range from familiar words like “optimistic” to rarer ones like “obstreperous”.
Below you’ll find the table with Word, Definition, Example sentence, and Register.
Word: The adjective itself, shown alphabetically so you can quickly scan for and select the exact term you need.
Definition: A concise one-line meaning that helps you understand the word’s sense and choose appropriate usage in context.
Example sentence: A short, natural example showing how native speakers use the adjective, helping you model correct phrasing.
Register: Labels like “formal”, “colloquial”, or “archaic” so you know when and where to use each adjective.
Adjectives that start with O
| Adjective | Register | Meaning | Example |
|---|---|---|---|
| oafish | informal | clumsy or rude in behavior | His oafish grip crushed the cup |
| oaken | made of oak wood | The table had a sturdy oaken top | |
| obese | very overweight, medically heavy | The clinic treats obese patients respectfully | |
| obedient | willing to follow orders or rules | The obedient dog sat and waited | |
| obdurate | formal | stubbornly refusing to change opinions | She remained obdurate despite all evidence |
| obese | duplicate omitted to avoid repetition | ||
| oblique | slanting; indirect or evasive | He gave an oblique answer to the question | |
| oblong | longer than wide; rectangularish | They found an oblong box in the attic | |
| obscene | offensively indecent or morally repulsive | The comic’s obscene jokes offended many | |
| obscure | not well known or hard to understand | The critic praised an obscure novelist | |
| observant | quick to notice things | She is observant during meetings | |
| obsolete | out of date or no longer used | That software is now obsolete | |
| obstinate | stubbornly refusing to change | The child remained obstinate about bedtime | |
| obstreperous | formal | noisy and difficult to control | The obstreperous students disrupted class |
| obsequious | formal | excessively eager to please or flatter | He disliked the obsequious assistant’s tone |
| obvious | easily perceived or understood | The solution was obvious to everyone | |
| occasional | happening from time to time | She enjoys an occasional glass of wine | |
| occult | formal | related to hidden, mystical knowledge | The old book described occult practices |
| occupational | formal | related to a job or occupation | He has an occupational hazard in his work |
| oceanic | formal | relating to the ocean | They studied oceanic currents and circulation |
| octagonal | having eight sides or angles | The gazebo had an octagonal roof | |
| odd | strange or not even in number | That’s an odd way to solve it | |
| odious | extremely unpleasant or hateful | They condemned the odious behavior | |
| odorless | without a smell | The gas was odorless and dangerous | |
| odorous | having a noticeable smell | The kitchen became odorous after cooking | |
| off | not operating, unpleasant, or away | The milk smells off; don’t drink it | |
| offbeat | informal | unconventional or quirky | Her offbeat style attracted readers |
| offensive | causing offense; aggressive | The remark was deeply offensive to many | |
| officious | overly eager to offer unwanted help | The officious clerk insisted on his way | |
| official | authorized or formal | The official report was released today | |
| ominous | suggesting something bad will happen | Dark clouds looked ominous before the storm | |
| ominous | duplicate omitted to avoid repetition | ||
| ominous | duplicate omitted | ||
| omnipotent | formal | having unlimited power | In myth, gods are often omnipotent |
| omniscient | formal | knowing everything | The narrator’s omniscient voice reveals thoughts |
| omnipresent | formal | present everywhere at once | Smartphones seem omnipresent in society |
| onerous | formal | burdensome or oppressive | They faced an onerous legal obligation |
| ongoing | continuing at present | The investigation is ongoing | |
| open | not closed; candid or available | The museum is open until nine | |
| operable | capable of being operated or used | The machine is operable after repairs | |
| operational | ready for use or working | The new system is fully operational | |
| operative | functioning; effective | The policy became operative last month | |
| opportune | well-timed; convenient | He made an opportune decision during crisis | |
| opportunistic | seizing chances often selfishly | His opportunistic habits annoyed colleagues | |
| opposite | completely different in nature | They hold opposite political views | |
| optimal | best or most favorable | We chose the optimal route home | |
| optional | left to choice; not required | Attendance at the event is optional | |
| opulent | formal | rich, luxurious, or lavish | They stayed in an opulent hotel suite |
| oral | spoken rather than written | She gave an oral presentation today | |
| orange | having the color orange | She wore an orange scarf to the party | |
| orderly | neat and well-arranged | The files were kept in an orderly stack | |
| ordinary | commonplace or not special | It was an ordinary weekday morning | |
| organic | relating to living matter or without chemicals | They buy organic vegetables at market | |
| original | first or novel; creative | Her original idea improved the design | |
| ornate | formal | richly decorated or elaborate | The ornate ceiling drew visitors’ attention |
| ornamental | formal | decorative rather than functional | The garden featured ornamental shrubs |
| orthodox | formal | conforming to traditional beliefs or practices | He follows orthodox teaching methods |
| ostensible | formal | appearing to be true but possibly false | The ostensible reason masked true motives |
| ostentatious | formal | showy to attract attention or impress | Her ostentatious jewelry felt excessive |
| other | different from the one mentioned | Take the other road to avoid traffic | |
| outer | located on the outside | Check the outer pocket for your keys | |
| outermost | farthest from the center | The outermost layer peeled away | |
| outgoing | friendly and sociable | He’s quite outgoing at parties | |
| outlandish | strange, unconventional, or bizarre | Her outlandish outfit turned heads | |
| outlook | duplicate omitted because noun not adjective | ||
| outspoken | bluntly expressive or candid | She is outspoken about workplace issues | |
| outstanding | exceptionally good or unresolved | Her performance was outstanding last night | |
| outrageous | shocking or wildly exaggerated | That claim is simply outrageous | |
| overcast | covered with clouds | It remained overcast all afternoon | |
| overall | taking everything into account; general | Overall, the project was a success | |
| overdue | past the expected time | The library book is overdue | |
| overjoyed | extremely happy | She was overjoyed at the news | |
| overnight | happening during the night or very quickly | The change seemed to happen overnight | |
| overt | open and observable; not hidden | They showed overt hostility in meetings | |
| overweight | weighing more than considered healthy | He was advised about overweight risks | |
| overripe | too ripe; past ideal ripeness | The banana was overripe for the recipe | |
| overworked | having too much work | The overworked staff needed a break | |
| overflowing | exceeding capacity and spilling over | The cup was overflowing with tea | |
| overwhelming | very intense or overpowering | They felt overwhelming relief after news | |
| oxblood | rare | deep brownish-red color | She chose an oxblood leather jacket |
| oxidized | technical | chemically combined with oxygen | The oxidized metal showed a green patina |
Descriptions
oafish
Informal, mildly insulting; suggests awkward, ungainly manners rather than malice
oaken
Traditional, descriptive; often used for furniture or timber tone
obese
Clinical/common; medical term for body mass much above healthy ranges
obedient
Common; implies compliance, often neutral or positive
obdurate
Formal/literary; stronger than “stubborn,” often morally negative
obese
oblique
Used physically (angles) and figuratively (indirect speech)
oblong
Common geometric adjective for stretched rectangles or shapes
obscene
Strong moral/sexual connotation; legal contexts too
obscure
Neutral: can mean hidden, unclear, or little-known
observant
Positive; implies attention to detail and awareness
obsolete
Common in tech and historical contexts; synonym: outdated
obstinate
Less formal than obdurate; often implies unreasonable persistence
obstreperous
Formal and somewhat literary; describes loud, unruly behavior
obsequious
Pejorative, denotes servility; synonyms: sycophantic
obvious
Very common; straightforward, may imply simplicity or lack of subtlety
occasional
Common; frequency adjective, less than frequent
occult
Used for supernatural or hidden arts; also medical (occult blood)
occupational
Common in formal contexts like health and law
oceanic
Scientific or descriptive; used in geography and science
octagonal
Geometric term, literal and architectural usage
odd
Very common; covers peculiarity and numerical oddness
odious
Strongly negative, literary tone; synonyms: hateful, detestable
odorless
Practical descriptor, often used in safety contexts
odorous
Neutral to negative; simply denotes smell presence
off
Informal; varied uses (not working, slightly wrong, away)
offbeat
Informal and positive; suggests creative oddness
offensive
Common; strong negative connotation, used in social evaluations
officious
Formal/pejorative; implies meddlesome, intrusive behavior
official
Used for sanctioned, formal statements or roles
ominous
Common in narrative; implies looming threat or bad sign
ominous
ominous
omnipotent
Elevated/formal; used figuratively to describe extreme power
omniscient
Literary term for all-knowing perspective
omnipresent
Figurative; widely used to stress ubiquity
onerous
Formal/legal tone; implies difficulty and unfair burden
ongoing
Common; neutral, used for processes or events in progress
open
Very common; broad uses—physical, emotional, opportunity
operable
Technical/medical use; contrasts with inoperable
operational
Common in business/technical contexts; implies functioning state
operative
Formal/legal/technical; can mean active or in effect
opportune
Formal, literary; implies timely appropriateness
opportunistic
Often pejorative; taking advantage without principle
opposite
Common; denotes direct contrast or facing position
optimal
Common in science and everyday use; synonym: ideal
optional
Very common; contrasts with mandatory
opulent
Formal and descriptive; used for wealth and luxury
oral
Common; used in education, exams, and communication contexts
orange
Color adjective; also fruit-related in compounds
orderly
Positive; implies organization and tidiness
ordinary
Very common; denotes lack of distinction
organic
Used in biology and food labeling; commercial term too
original
Common; can mean authentic, first, or inventive
ornate
Formal/style; suggests complexity and decorative richness
ornamental
Used for decorative, non-essential elements
orthodox
Religious, cultural, or methodological conformity suggested
ostensible
Formal; implies a surface explanation hides reality
ostentatious
Negative; flashy display meant to impress others
other
Very common; basic comparative/differentiation term
outer
Spatial adjective; contrasts with inner or innermost
outermost
Used spatially; denotes extreme outer position
outgoing
Personality adjective; positive, describes sociability
outlandish
Often colorful or eccentric; can be humorous or critical
outlook
outspoken
Describes frank speech; can be admired or criticized
outstanding
Common; praise or denotes unresolved debts/tasks
outrageous
Strong negative or emphatic; often about behavior or costs
overcast
Weather adjective; implies dull, cloudy sky
overall
Common as adjective/adverb; summarizes or gives general view
overdue
Common; used for payments, deadlines, arrivals
overjoyed
Strong positive emotion; often temporary
overnight
Adjective or adverb; can mean suddenly or during the night
overt
Opposite of covert; used in politics and behavior
overweight
Common medical/descriptor; less clinical than obese
overripe
Used for fruit/produce and metaphorically for timing
overworked
Adjective established in dictionaries; describes excessive workload
overflowing
Often literal; also figurative for abundance
overwhelming
Common; used for feelings, odds, evidence
oxblood
Color adjective, used in fashion and design contexts
oxidized
Mostly technical/scientific; also used metaphorically for aging metal