52 Nouns That Start With "O"
52 curated nouns that start with "O".
Below are 52 nouns that start with "O" in our curated English dictionary. Each entry includes a learner-friendly definition, IPA pronunciation, CEFR difficulty level, and example sentences. Words are sorted by frequency — the most common first. Click any word to view its full dictionary entry with synonyms, collocations, and common mistakes.
Most Common Nouns That Start With "O" (Top 1k)
Less Common Nouns That Start With "O" (Top 5k+)
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Observation /ˌɒbzərˈveɪʃən/noun B1
When you notice and watch something carefully to understand it.
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Ocean /ˈoʊʃən/noun A1
The largest saltwater environment on Earth with waves and marine life.
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Onion /ˈʌnjən/noun A1
A vegetable that makes you cry, used in cooking.
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Order /ˈɔːrdər/noun A1
A request asking someone to make, deliver, or sell you something.
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Oven /ˈʌvən/noun A1
A hot box where you cook food, like baking cookies or roasting chicken.
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Owner /ˈoʊnər/noun A1
The person that something belongs to.
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Oar /ɔːr/noun B1
A stick with a flat end that rowers use to move boats forward.
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Object /ˈɒbdʒɪkt/noun A1
Any item or thing you can hold or see.
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Occasion /əˈkeɪʒən/noun A2
A special event or moment when something takes place.
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Octopus /ˈɒk.tə.pʊs/noun B1
A soft sea animal with eight arms and no bones, able to change its color.
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Offense /əˈfɛns/noun B1
An action that breaks the law or rules; a crime.
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Office /ˈɔːfɪs/noun A1
A place where people work, usually with desks and computers.
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Officer /ˈɔːfɪsər/noun A2
A person with an official position and responsibility; often in police or military.
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Offshoring /ˌɔːfˈʃɔːrɪŋ/noun B2
When a company moves its operations or factories to another country, usually to save money on labor costs.
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Opening /ˈoʊpənɪŋ/noun A2
A space where something is open; or the beginning of an event.
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Opinion /əˈpɪnjən/noun A1
What you think or believe about something; your personal view or judgment.
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Option /ˈɑːpʃən/noun A1
One of several things you can choose or pick from.
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Orange /ˈɑːrɪndʒ/noun A1
A round fruit that is orange in color and tastes sweet and sour.
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Oscillation /ˌɒsɪˈleɪʃən/noun B2
When something moves or changes back and forth in a steady, regular pattern.
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Outcome /ˈaʊtkʌm/noun B1
The final result of something that happens. What ends up happening after something occurs.
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Outline /ˈaʊtlaɪn/noun B1
The shape you see around the edge of something. Or a short plan or summary showing main ideas.
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Outsourcing /ˈaʊtˌsɔːrsɪŋ/noun B2
When a company hires another company to do work instead of doing it themselves inside the organization.
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Overhead /ˈoʊvərˌhed/noun B2
The general costs of running a business that aren't tied to making specific products. Things like paying for the office, electricity, and managers' salaries.
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Oxygen /ˈɑːksɪdʒən/noun A2
The gas in air that living things need to breathe.
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Oasis /oʊˈeɪ.sɪs/noun B2
A green, watery spot in the middle of a desert where plants and people can survive.
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Objection /əbˈdʒɛkʃən/noun B1
A statement saying you do not agree with something.
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Obsession /əbˈseʃən/noun B2
When you think about something or someone so much that you cannot stop, even if you try. It takes over your mind and life.
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Oligarchy /ˈɑːlɪɡɑːrki/noun C2
A system where a few powerful people control everything. It could be a small group of rich people, a few powerful families, or a small military elite. These few people make all the decisions for everyone else.
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Onset /ˈɑːnset/noun C1
When something starts or begins, particularly a disease, problem, or difficult period. The moment it all begins.
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Operation /ˌɑpəˈreɪʃən/noun B1
An action or activity; especially a medical procedure done by a surgeon in a hospital.
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Opponent /əˈpoʊnənt/noun B1
The person or team that you are playing against in a sport or competition.
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Opportunity /ˌɑpərˈtuːnəti/noun B1
A good chance or moment when something can be done; a favorable situation.
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Optimizer /ˈɑːptɪmaɪzər/noun B2
A tool or method that makes something work better or faster.
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Ordinance /ˈɔːrdɪnəns/noun C1
A local law made by a city or town government. Similar to a statute but usually applies to a specific area or community.
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Orientation /ˌɔːrɪənˈteɪʃən/noun C1
The direction something faces or points. Or training you receive when you start a new job or school. Or someone's attitudes and interests.
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Ourselves /aʊərˈsɛlvz/noun A2
A word used when a group including you does something to or for themselves.
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Output /ˈaʊtpʊt/noun B2
How much something produces or makes. In computers, it's the information a machine gives you as a result.
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Outset /ˈaʊtset/noun C1
The very beginning of something you plan to do or something that is happening. The first moment when it starts.
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Overdraft /ˈoʊvərˌdræft/noun B2
When you take out more money than you have in your account. The bank may let you do this, and you pay it back with interest.
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Overfitting /ˌoʊvərˈfɪtɪŋ/noun B2
When a computer model memorizes training examples instead of learning general rules.
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Ownership /ˈoʊnərʃɪp/noun B1
The condition of possessing and controlling something legally.
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Oyster /ˈɔɪ.stər/noun B1
An edible marine mollusk with two hard shells that protect soft tissue inside.
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Ozone /ˈoʊzoʊn/noun B2
A special type of oxygen gas with three atoms instead of two. High up it protects us from sun damage, but near the ground it pollutes the air we breathe.
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Ode /oʊd/noun C2
A praise poem that celebrates something or someone; a formal, often lengthy poem expressing admiration.
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Omakase /ˌoʊməˈkɑːsi/noun C2
A Japanese dining style where the chef chooses what to serve you, based on freshest ingredients and chef's expertise.
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Onomatopoeia /ˌɒnəˌmætəˈpiːə/noun C2
Sound imitation through word choice
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Ontology /ɑːnˈtɑːlədʒi/noun C2
The philosophical study of what exists and what it means to exist; the study of being and existence.
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Oxymoron /ɒkˈsɪmɒrɒn/noun C2
Juxtaposition of contradictions
How to Use Nouns That Start With "O"
Nouns that start with "O" can act as the subject, object, or complement of a sentence. Pay attention to whether each noun is countable or uncountable — countable nouns take "a/an" and have plural forms (cat → cats), while uncountable nouns (water, advice) do not. The CEFR badge on each card tells you when learners typically meet the word; A1-A2 nouns are essential for everyday conversations, B1-B2 are common in news and writing, and C1-C2 nouns appear in academic and specialised texts.
Longest Noun That Start With "O"
A company, club, or group working together for common goals.
Frequently Asked Questions About Nouns That Start With "O"
- How many nouns that start with "O"?
- Our curated dictionary lists 52 nouns that start with "O", each with a definition, IPA, CEFR level, and example sentences. The list grows as new entries are reviewed.
- What are the most common nouns that start with "O"?
- Some of the most frequent nouns that start with "O" include OIL, ONE, ORGANIZATION, ORIGIN. These appear in everyday English and are useful for both beginners (A1-A2) and intermediate learners.
- What is noun?
- A noun is a word for a person, place, thing, idea, or quality. Nouns are the building blocks of every sentence and act as subjects, objects, or complements.
- What is the longest noun that start with "O"?
- The longest noun that start with "O" in our dictionary is "ORGANIZATION" (12 letters). Tap the word to see its full entry.