This list includes 86 Nouns that start with O, from “oak” to “ozone”. These entries are mostly common, everyday nouns useful for writing, teaching, and vocabulary study.
Nouns that start with O are words that name people, places, things, or ideas beginning with the letter O. Many derive from Latin, Old English, or other languages, and include familiar loanwords like “oasis.”
Below you’ll find the table with Noun, Definition, and Example.
Noun: The base word in its singular form, so you can spot and learn each entry quickly.
Definition: A concise one-line meaning that helps you understand common usage and differences between similar nouns.
Example: A short natural sentence showing how you can use the noun in everyday speech or writing.
Nouns that start with O
| Noun | Plural | Countability | Example |
|---|---|---|---|
| oasis | oases | countable | After days of walking, they found a small oasis. |
| oar | oars | countable | He grabbed the broken oar and started rowing immediately. |
| oak | oaks | countable | A huge oak stood near the edge of the field. |
| oath | oaths | countable | She took an oath to tell the whole truth. |
| object | objects | countable | The curious child picked up the small object quickly. |
| objective | objectives | countable | Our main objective is to finish the project by Friday. |
| obligation | obligations | countable | I have a legal obligation to report that information. |
| observation | observations | countable | Her observation about the data surprised everyone at the meeting. |
| observer | observers | countable | An independent observer recorded every vote at the count. |
| obstacle | obstacles | countable | They faced a major obstacle on the hiking trail today. |
| occasion | occasions | countable | This is a perfect occasion to celebrate your promotion tonight. |
| occupation | occupations | countable | Her occupation as a teacher suits her calm personality. |
| ocean | oceans | countable | We sailed across the ocean last summer with friends. |
| octagon | octagons | countable | The table had an octagon shape that fit the corner. |
| octopus | octopuses | countable | An octopus changed color while hiding beneath the rock. |
| ode | odes | countable | She wrote an ode to her grandmother’s kindness. |
| odometer | odometers | countable | The car’s odometer showed over 100,000 miles driven. |
| odor | odors | countable | A strange odor came from the old refrigerator overnight. |
| officer | officers | countable | A police officer stopped to help the frightened child. |
| office | offices | countable | She left the office early to pick up groceries. |
| offer | offers | countable | They made a generous offer to buy the small company. |
| offspring | — | countable | The deer watched over its newborn offspring in the meadow. |
| oil | — | uncountable | Add two tablespoons of oil to the hot pan. |
| olive | olives | countable | She picked a ripe olive from the summer branch. |
| olive oil | — | uncountable | Drizzle some olive oil over the warm bread before serving. |
| omelet | omelets | countable | I cooked a cheese omelet for breakfast this morning. |
| onion | onions | countable | Chop one onion finely before adding it to soup. |
| opening | openings | countable | There is a job opening in the marketing department now. |
| opera | operas | countable | We attended a famous opera at the city theater Saturday. |
| operation | operations | countable | The hospital scheduled the operation for next Tuesday morning. |
| operator | operators | countable | The elevator operator greeted each passenger with a smile. |
| opinion | opinions | countable | In my opinion, the film deserved the award tonight. |
| opponent | opponents | countable | Her strongest opponent refused to debate next week. |
| opportunity | opportunities | countable | This internship offers a great opportunity to learn new skills. |
| order | orders | countable | They placed an order for ten new chairs yesterday. |
| organ | organs | countable | The church organ played a soft melody during the ceremony. |
| organization | organizations | countable | The charity organization provided food and shelter after floods. |
| organizer | organizers | countable | The event organizer confirmed the schedule with all vendors. |
| origin | origins | countable | Scientists traced the origin of the artifact to ancient times. |
| ornament | ornaments | countable | She hung a glass ornament on the Christmas tree. |
| orphan | orphans | countable | The orphan found a loving home with a new family. |
| orchestra | orchestras | countable | The orchestra rehearsed the concerto for several evenings. |
| orchid | orchids | countable | An orchid bloomed on the windowsill despite the cold weather. |
| ordeal | ordeals | countable | Recovering from the accident was a long, difficult ordeal. |
| ore | — | uncountable | Miners extracted ore from the mountain for months. |
| outcome | outcomes | countable | The outcome of the vote surprised everyone at council. |
| outlet | outlets | countable | She opened a new clothing outlet near the university. |
| outline | outlines | countable | Please send an outline of your proposal by Friday. |
| outlook | outlooks | countable | His optimistic outlook helped the team through hard times. |
| outlaw | outlaws | countable | The film portrayed him as a sympathetic outlaw on the run. |
| ounce | ounces | countable | Add one ounce of vanilla extract to the recipe. |
| ovation | ovations | countable | She received a standing ovation after the final scene. |
| oven | ovens | countable | Preheat the oven to 375 degrees before baking. |
| overdose | overdoses | countable | He was rushed to hospital after an accidental overdose. |
| overview | overviews | countable | The manager gave an overview of the quarterly results. |
| owl | owls | countable | A barn owl hunted quietly outside the farmhouse last night. |
| owner | owners | countable | The store owner locked up early due to rain. |
| ownership | — | uncountable | After years of work, she finally secured ownership of the house. |
| ox | oxen | countable | A team of ox pulled the heavy cart through mud. |
| oxygen | — | uncountable | Plants produce oxygen during photosynthesis on sunny summer days. |
| oatmeal | — | uncountable | She stirred cinnamon into the warm oatmeal before serving. |
| oboe | oboes | countable | The oboe solo added a haunting tone to the piece. |
| obituary | obituaries | countable | The family wrote a brief obituary for the local newspaper. |
| obelisk | obelisks | countable | A tall obelisk stood in the center of the plaza. |
| opal | opals | countable | She wore a necklace with a small opal stone. |
| opossum | opossums | countable | An opossum scavenged through the backyard late last night. |
| oracle | oracles | countable | The ancient oracle gave cryptic advice to the pilgrims. |
| orator | orators | countable | The orator delivered a passionate speech to the crowd. |
| organism | organisms | countable | Each organism in the pond plays a role in balance. |
| oregano | — | uncountable | Sprinkle dried oregano over the pizza before baking it. |
| orphanage | orphanages | countable | They donated clothing and books to the local orphanage. |
| oval | ovals | countable | The painting had an oval frame that suited the portrait. |
| ovary | ovaries | countable | Doctors monitored her ovary during the fertility treatment. |
| overcoat | overcoats | countable | He put on his wool overcoat before leaving in rain. |
| overtime | — | uncountable | She earned overtime after working extra hours all week. |
| overthrow | overthrows | countable | The rebels planned an overthrow of the corrupt government secretly. |
| omission | omissions | countable | His omission of details changed the tone of the report. |
| onslaught | onslaughts | countable | The storm felt like an onslaught against the coastal town. |
| ostrich | ostriches | countable | An ostrich ran across the field faster than I expected. |
| ozone | — | uncountable | The ozone layer protects life from harmful ultraviolet radiation. |
| opium | — | uncountable | He studied the history of opium trade in Asia. |
| oxbow | oxbows | countable | We visited an oxbow where the river had curved away. |
| orientation | orientations | countable | The new employee attended orientation on her first day. |
| orifice | orifices | countable | Water dripped from a small orifice in the pipe. |
| oratorio | oratorios | countable | The choir prepared an oratorio for the holiday concert next month. |
| ordinance | ordinances | countable | The city passed a new ordinance about noise after midnight. |
Descriptions
oasis
A small fertile area in a desert where water and plants appear.
oar
A long pole with a flat blade used to row or steer a boat.
oak
A large, long-lived hardwood tree or its wood, common in parks.
oath
A formal promise, often made in court or for official duties.
object
A thing that can be seen or touched; a physical item.
objective
A goal or aim that someone is trying to achieve.
obligation
A duty or responsibility someone must fulfill.
observation
A note or comment based on seeing or studying something.
observer
A person who watches or monitors an event or process.
obstacle
Something that blocks progress or makes it difficult to do something.
occasion
A particular event or time when something happens.
occupation
A person’s job or the way someone spends their time.
ocean
A very large body of salt water covering most of Earth.
octagon
A polygon with eight sides; also anything shaped like it.
octopus
A sea animal with eight arms known for intelligence and camouflage.
ode
A short lyrical poem praising something or someone.
odometer
An instrument in a vehicle that measures distance traveled.
odor
A smell, often noticeable and sometimes unpleasant.
officer
A person with a position of authority in the police, military, or organization.
office
A place where people work at desks or where official business is done.
offer
A proposal to give something or to do something for someone.
offspring
The young of an animal or the result/children of a person or couple.
oil
A viscous liquid used for cooking, machines, or as a fuel or lubricant.
olive
A small fruit often used for oil and cooking, and a tree species.
olive oil
Oil pressed from olives, commonly used in cooking and dressings.
omelet
A folded egg dish often mixed with fillings like cheese or vegetables.
onion
A bulb vegetable used to flavor many dishes.
opening
A vacancy, gap, or opportunity; a place where something begins.
opera
A dramatic musical performance combining singing, orchestra, and stagecraft.
operation
A medical surgical procedure; also an organized activity or process.
operator
A person who controls equipment or runs a machine, service, or system.
opinion
A personal belief or judgment that is not necessarily a fact.
opponent
Someone competing against another in a contest, argument, or election.
opportunity
A favorable chance or situation for advancement or progress.
order
A request to buy something; a sequence or arrangement; a command.
organ
A musical keyboard instrument with pipes; also a body part with a function.
organization
A group formed for a particular purpose, like charity or business.
organizer
A person who plans and arranges events or activities.
origin
The beginning, source, or place where something started.
ornament
A decorative object used to beautify something, often seasonal.
orphan
A child who has lost one or both parents; used figuratively sometimes.
orchestra
A large group of musicians playing different instruments together.
orchid
A diverse and often showy flowering plant, popular as a houseplant.
ordeal
A very unpleasant or painful experience that tests someone.
ore
Natural rocks or minerals from which metals or valuable materials can be obtained.
outcome
The result or consequence of an action, event, or process.
outlet
A store, distribution point, or means of expressing ideas or emotions.
outline
A general description or plan giving the main points of something.
outlook
A person’s general attitude or a likely future situation or view.
outlaw
A person who has broken the law and is hiding or pursued.
ounce
A common unit of weight (oz) or a small amount of something.
ovation
Enthusiastic applause or acclaim from an audience.
oven
A kitchen appliance or enclosed space used for baking or roasting food.
overdose
Taking an excessive and dangerous amount of a drug or substance.
overview
A brief summary that gives the main points of a subject.
owl
A nocturnal bird of prey with large eyes and a rotating head.
owner
A person who legally possesses something, like a business or property.
ownership
The state of owning something; legal right to possess property.
ox
A large domesticated bovine animal, often used as a beast of burden.
oxygen
A chemical element essential for respiration and combustion, a gas in air.
oatmeal
A soft hot cereal made from oats, commonly eaten for breakfast.
oboe
A woodwind instrument with a double reed, used in orchestras.
obituary
A published notice of a person’s death, often with a short biography.
obelisk
A tall, four-sided, tapering monument that ends in a pyramid shape.
opal
A gemstone known for its colorful play of light.
opossum
A nocturnal marsupial found in the Americas, known for playing dead.
oracle
A prophetic figure or source of wise or authoritative guidance.
orator
A skilled public speaker who delivers formal speeches.
organism
A living thing, such as an animal, plant, fungus, or microbe.
oregano
A common culinary herb used fresh or dried to flavor food.
orphanage
A residential institution caring for children without parents.
oval
An egg-shaped curved geometric shape or object.
ovary
A female reproductive organ that produces eggs or seeds in plants.
overcoat
A long warm coat worn over other clothing in cold weather.
overtime
Time worked beyond the regular working hours; pay often increases.
overthrow
The act of removing a government or leader by force or coup.
omission
Something left out or neglected, intentionally or accidentally.
onslaught
A fierce or destructive attack or large-scale aggressive action.
ostrich
A very large flightless bird native to Africa, known for speed.
ozone
A form of oxygen in the atmosphere that absorbs ultraviolet light.
opium
A narcotic substance obtained from poppies, historically significant in trade and medicine.
oxbow
A U-shaped bend in a river, sometimes forming a separate lake.
orientation
An introductory session to help someone learn procedures or a new place.
orifice
An opening or hole in a surface or body structure.
oratorio
A large musical composition for orchestra and voices, usually on religious themes.
ordinance
A law or regulation made by a city or town government.