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C2 noun /ɑːlˈfækʃən/

olfaction

Meaning
The sense of smell; the process of detecting and perceiving odors.
Example
Dogs have a highly developed sense of olfaction.
C1 noun /ˈɑn.set/

Onset

Meaning
the beginning or early stages of something; the start of a medical condition
Example
The onset of winter brought heavy snowfall.
C1 adjective /ˈaʊt.kɑːst/

Outcast

Meaning
rejected by society; excluded person
Example
He felt like an outcast in his new school.
C1 verb /əbˈsɛs/

obsess

Meaning
To preoccupy or fill the mind of someone continually and to an excessive degree.
Example
She tends to obsess over small details.
C1 verb /ˌaʊtˈstrɪp/

outstrip

Meaning
To go faster or do better than someone or something; to surpass.
Example
The company outstripped its competitors in sales last year.
C2 verb ˈɑːb.fə.skeɪt

obfuscate

Meaning
Render obscure, unclear, or unintelligible.
Example
Politicians often obfuscate facts to mislead people.
B2 verb /ˌoʊvərˈduː/

overdo

Meaning
to do something too much; to exaggerate or carry to excess
Example
Don't overdo the exercise on your first day at the gym.
B2 noun /ˈɒs.trɪtʃ/

Ostrich

Meaning
a large flightless bird native to Africa with long legs and neck
Example
The ostrich is the world's largest bird.
B2 noun /ˈoʊ.dər/

Odour

Meaning
a distinctive smell, especially an unpleasant one
Example
The odour of fresh bread filled the kitchen.
C1 noun /ˈɒktɪv/

octave

Meaning
A series of eight musical notes, or the interval between one musical pitch and another with double its frequency.
Example
The singer's voice could easily reach the higher octave.
C2 adjective /ˈɒfɪʃ/

offish

Meaning
Unfriendly, distant, or reserved in manner.
Example
He seemed offish at the party and avoided most conversations.
C1 adjective /əˈblɪɡ.ə.tɔː.ri/

obligatory

Meaning
Required by a legal, moral, or other rule; compulsory.
Example
Wearing a seatbelt is obligatory in most countries.
B2 noun /ˈaʊt.kʌm/

Outcome

Meaning
the result or consequence of an action or situation; the final product
Example
The outcome of the election surprised everyone.
C1 verb /uːz/

ooze

Meaning
to slowly flow or leak out through small openings or pores
Example
Blood began to ooze from the small cut.
C2 verb /ˈɔːreɪt/

orate

Meaning
To speak in a formal, often pompous manner
Example
He began to orate at the ceremony.
C2 adjective /ɔːrˈθɑːɡənl/

orthogonal

Meaning
at right angles; independent or unrelated
Example
In mathematics, two vectors are orthogonal if their dot product is zero.
B2 verb /əbˈteɪn/

obtain

Meaning
to get or acquire something
Example
You need to obtain a visa before traveling abroad.
C1 adjective /oʊˈbiːs/

obese

Meaning
Extremely overweight; having an excessive amount of body fat.
Example
The doctor advised the obese patient to adopt a healthier lifestyle.
C2 adjective /ɒmˈnɪp.ə.tənt/

omnipotent

Meaning
Having unlimited power; able to do anything; all-powerful.
Example
In many religions, God is considered omnipotent.
C2 noun /əbˈstɛtrɪks/

obstetrics

Meaning
The branch of medicine dealing with childbirth and care of women giving birth.
Example
She is studying obstetrics to become a specialist in childbirth.
C2 adjective /ˈɑːsɪˌfaɪd/

ossified

Meaning
Turned into bone or become rigid; unwilling to change.
Example
The company had an ossified management structure that resisted innovation.
A2 adjective /ˈaʊtdɔːr/

outdoor

Meaning
situated, existing, or done outside a building
Example
We planned an outdoor picnic for the weekend.
B1 verb /əbˈzɜːrv/

observe

Meaning
to watch carefully; to notice or perceive something
Example
Scientists observe the behavior of animals in the wild.
C2 noun /ˌɒn.əˌmæt.əˈpiː.ə/

onomatopoeia

Meaning
The formation of a word from a sound associated with what is named.
Example
The word 'buzz' is an example of onomatopoeia.
B1 adjective /ˈɔːɡənaɪzd/

organized

Meaning
Arranged in a systematic way, especially on a large scale.
Example
An organized approach to work increases productivity and reduces stress.
C1 noun ˌɒk.sɪˈdeɪ.ʃən

oxidation

Meaning
A chemical reaction that involves the combination of a substance with oxygen, often causing deterioration.
Example
Oxidation causes metal surfaces to weaken.
C1 noun /ˈɔr.i.ənt/

Orient

Meaning
the East, especially countries of eastern Asia; to determine one's position or direction
Example
Many spices come from the Orient.
B2 noun /ˈɒkjʊpənt/

occupant

Meaning
A person who resides in or is present in a house, vehicle, seat, or place.
Example
The car had three occupants at the time of the accident.
C2 adjective /ˌoʊdəˈrɪfərəs/

odoriferous

Meaning
having or giving off a smell, especially a strong or pleasant one
Example
The garden was filled with odoriferous flowers.
C1 noun /ˈɔr.ə.kəl/

Oracle

Meaning
a person regarded as a source of wise counsel; in ancient times, a priest or priestess acting as a medium
Example
The ancient Greeks consulted the oracle at Delphi.
B1 adjective /ˌoʊld ˈfæʃənd/

old-fashioned

Meaning
not modern; belonging to an earlier time
Example
He still wears an old-fashioned hat.
C1 verb /ɔːrˈdeɪn/

ordain

Meaning
To officially make someone a priest or minister; to decree or order something officially.
Example
The bishop will ordain three new priests this Sunday.
B1 noun /ˌɒpəˈreɪʃən/

operation

Meaning
an act or process of functioning or operating
Example
The hospital performed a successful operation.
C1 verb /aʊtˈlɪv/

outlive

Meaning
to live longer than someone or something else
Example
She outlived all her siblings.
B2 adjective /ˈɒp.ʃən.əl/

Optional

Meaning
not compulsory; left to choice
Example
Wearing a tie is optional for this event.
C2 noun /ˈɔːfəl/

offal

Meaning
the internal organs and entrails of an animal used as food; waste parts
Example
The butcher prepared sausages using pork offal.
A2 noun /əˈlɪmpɪks/

olympics

Meaning
an international sports competition held every four years with athletes from many countries
Example
The Olympics bring athletes from all over the world together.
A2 noun /ˈɒm.lət/

Omelet

Meaning
a dish made from beaten eggs cooked in a frying pan and often folded around a filling
Example
She made a cheese omelet for breakfast.
C2 adjective /ˈɒpjʊlənt/

opulent

Meaning
Ostentatiously rich, luxurious, or lavish.
Example
They hosted an opulent wedding with gold decorations.
B2 noun /ˈoʊtˌmiːl/

oatmeal

Meaning
A type of porridge made from ground oats, often eaten for breakfast.
Example
She eats a bowl of oatmeal every morning for breakfast.
C1 noun /ˌɑːsɪˈleɪʃən/

oscillation

Meaning
The movement back and forth at a regular speed.
Example
The oscillation of the pendulum controls the clock.
B1 noun /əˈpɪn.jən/

Opinion

Meaning
a view or judgment formed about something; a belief or assessment based on grounds short of proof
Example
In my opinion, this is the best solution.
A2 adjective /ˈɔː.dɪ.nər.i/

Ordinary

Meaning
normal; usual; not special
Example
It was just an ordinary day at the office.
B2 noun /əˈfɛns/

offense

Meaning
an illegal act; a crime
Example
The offense was committed in broad daylight.
B2 noun/verb /ˌoʊvərˈlæp/

overlap

Meaning
To extend over and cover partly; an area where things coincide.
Example
The two meetings overlap by an hour.
C2 noun /ˈɒfərtɔːri/

offertory

Meaning
The part of a religious service when offerings are collected; the offering itself.
Example
The offertory was taken during the Sunday service.
C1 noun /ˈoʊvərˌsiːər/

overseer

Meaning
a person who supervises work or workers; a manager
Example
The overseer made sure the workers followed safety rules.
B2 noun /ˈɑp.tə.mɪst/

Optimist

Meaning
a person who tends to be hopeful and confident about the future; someone who sees the good side of things
Example
As an optimist, she always believed things would improve.
A2 modal verb /ɔːt/

ought

Meaning
Used to indicate duty, correctness, or moral obligation.
Example
You ought to respect your elders.
C1 verb /ˌaʊtˈweɪ/

outweigh

Meaning
To be more important, valuable, or significant than something else.
Example
The benefits of the new policy outweigh the risks.
C1 noun ˌoʊ.vɚˈɡreɪ.zɪŋ

overgrazing

Meaning
The practice of allowing livestock to graze to the point of damaging vegetation.
Example
Overgrazing leads to soil erosion and loss of biodiversity.
A2 noun əˈlɪm.pɪk ɡeɪmz

Olympic Games

Meaning
The world's largest international sports competition held every four years, bringing together athletes from different countries to compete in various sports.
Example
The Olympic Games bring together the world's top athletes.
B2 adverb /ˌoʊvərˈnaɪt/

Overnight

Meaning
during the night; suddenly; very quickly
Example
The city was transformed overnight.
C1 verb /əˈfɪʃieɪt/

officiate

Meaning
To perform official duties, especially in a ceremony or as a referee.
Example
The priest will officiate the wedding ceremony.
B2 verb /ˌoʊvərˈwɛlm/

overwhelm

Meaning
to overpower someone emotionally or physically; to be too much to handle
Example
She was overwhelmed with gratitude after receiving the help.
A2 noun /ˈoʊ.nər/

Owner

Meaning
a person who owns something; someone who has legal possession of property
Example
The owner of the restaurant greeted us warmly.
C2 noun /ˌɔːrnɪˈθɑːlədʒi/

ornithology

Meaning
the scientific study of birds
Example
She pursued ornithology to understand the migration patterns of birds.
C1 noun /ˈɒf.sprɪŋ/

offspring

Meaning
a person's child or children; an animal's young
Example
The bird carefully fed its offspring.
C2 adjective /ɑːlˈfæktəri/

olfactory

Meaning
Relating to the sense of smell.
Example
The perfume activated her olfactory nerves immediately.
C2 adjective /əˈprəʊ.bri.əs/

opprobrious

Meaning
Expressing strong criticism or scorn.
Example
He made opprobrious remarks about his opponent.
C2 noun /ˌɔːrəˈtɔːri.oʊ/

oratorio

Meaning
A large-scale musical composition for orchestra, choir, and soloists, usually on a sacred theme, performed without costumes or scenery.
Example
Handel's 'Messiah' is one of the most famous oratorios ever written.
C1 noun /ˈoʊvərˌtoʊn/

overtone

Meaning
A subtle quality, implication, or additional meaning beyond the obvious.
Example
His speech had political overtones that were hard to ignore.
B2 noun /əbˈzɜːvər/

observer

Meaning
a person who watches or notices something
Example
The observer took notes during the meeting.
A2 adverb /ˈɒbviəsli/

obviously

Meaning
in a way that is easily perceived or understood; clearly
Example
Obviously, he didn’t read the instructions.
B2 noun ˈəʊnəʃɪp

ownership

Meaning
The act, state, or right of possessing something; legal title to something.
Example
Ownership of resources was crucial.
A1 pronoun /ˈʌðərz/

others

Meaning
people or things distinct from the ones already mentioned or known
Example
I like to spend time with others.
C1 adjective /ˈɒptɪməm/

optimum

Meaning
Most conducive to a favorable outcome; best.
Example
To achieve optimum results, we need to improve efficiency.
C2 adjective /əbˈsiːkwɪəs/

obsequious

Meaning
Excessively obedient or attentive in a servile way.
Example
The waiter gave an obsequious bow to every customer.
B2 verb /ˌoʊvərˈiːt/

overeat

Meaning
To eat more food than the body needs.
Example
She tends to overeat when she is stressed.
A1 verb, adjective /oʊn/

own

Meaning
to possess something; belonging to oneself
Example
She owns a beautiful house in the city.
C2 noun /ˈoʊvərlɔːrd/

overlord

Meaning
A ruler, master, or person of great power and authority over others.
Example
The peasants were forced to pay taxes to their overlord.
C1 noun ˈoʊʃən əˌsɪdɪfɪˈkeɪʃən

ocean acidification

Meaning
The ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide from the atmosphere.
Example
Ocean acidification is caused by atmospheric CO2 absorption.
C1 noun /əbˈzɜːrvətɔːri/

observatory

Meaning
A building equipped for observing astronomical or meteorological events.
Example
The observatory is open to the public on weekends.
B2 adjective + noun ˈɒptɪməl ˈtempərətʃər

optimal temperature

Meaning
The best or most favorable temperature for a particular purpose
Example
Winter provides optimal temperature for farming.
C2 adjective /ˈɑːbdjʊrət/

obdurate

Meaning
Stubbornly refusing to change one's opinion or course of action.
Example
He remained obdurate despite everyone's pleas.
C2 adjective /əbˈstrɛpərəs/

obstreperous

Meaning
Noisy and difficult to control.
Example
The obstreperous students disrupted the class.
B1 adverb /əˈkeɪʒənəli/

Occasionally

Meaning
sometimes; from time to time; not regularly
Example
I occasionally visit my grandmother on weekends.
C2 verb /ˈɑːbvieɪt/

obviate

Meaning
To remove a need or difficulty; to prevent or avoid something.
Example
This new system obviates the need for manual data entry.
B2 noun /əbˈdʒek.ʃən/

Objection

Meaning
an expression of disagreement or disapproval; a reason for disagreeing
Example
The lawyer raised an objection during the trial.
C1 adjective /ˌɔː.riˈen.təl/

Oriental

Meaning
relating to the countries of East Asia; eastern
Example
The museum has a beautiful collection of oriental art.
C1 noun /ˈaʊtˌkraɪ/

outcry

Meaning
A strong expression of public anger or disapproval.
Example
The decision sparked an outcry among the citizens.
A1 adverb /wʌns/

Once

Meaning
one time only; at some time in the past; formerly
Example
I have been to Paris once in my life.
B2 noun /ˈɔr.fən.ɪʒ/

Orphanage

Meaning
an institution for the care and education of children who have lost their parents
Example
The charity built a new orphanage for homeless children.
C1 noun /ˈoʊ.mən/

Omen

Meaning
an event regarded as a sign of future good or bad luck
Example
Many people consider a black cat crossing your path a bad omen.
B2 adjective /ˌoʊvərˈwɛlmɪŋ/

overwhelming

Meaning
Very intense; overpowering.
Example
Large tasks can feel overwhelming.
B1 noun /ˈɑb.stə.kəl/

Obstacle

Meaning
something that blocks or makes progress difficult; a barrier
Example
Bad weather was the main obstacle to our journey.
A2 noun /ˈɒbdʒɪkt/

object

Meaning
a thing that can be seen and touched
Example
The table is an object in the room.
C1 adjective /ˌaʊt əv ðə ˈweɪ/

out-of-the-way

Meaning
remote or unusual; not in a convenient or usual location
Example
They stayed in an out-of-the-way village.
C2 noun /oʊd/

ode

Meaning
a lyrical poem, often expressing praise or strong emotions
Example
The poet wrote an ode to the beauty of nature.
C1 verb /ˈɒfsɛt/

offset

Meaning
To counterbalance or compensate for something.
Example
The company's losses were offset by gains in other departments.
A1 pronoun, noun, adjective /wʌn/

one

Meaning
the number equivalent to the sum of zero and one; a single person or thing
Example
Only one student answered the question.
C1 noun /oʊˈmɪʃ.ən/

Omission

Meaning
the action of excluding or leaving out someone or something; a failure to include
Example
The omission of his name from the list was intentional.
B2 noun /ˈɔr.fən/

Orphan

Meaning
a child whose parents are dead; someone without protective support
Example
The orphan was adopted by a loving family.
C2 adjective /ˌɒbsəˈlɛsnt/

obsolescent

Meaning
Becoming outdated or on the way to becoming obsolete.
Example
The obsolescent technology could not compete with modern devices.
C2 noun /ˈɒb.stɪ.nə.si/

obstinacy

Meaning
Stubborn refusal to change one's opinion or chosen course of action, despite attempts to persuade one to do so.
Example
His obstinacy made negotiations difficult.
C1 verb /ˌoʊvərˈpaʊər/

overpower

Meaning
To defeat or gain control over someone or something by using greater strength or force.
Example
The soldiers managed to overpower the enemy forces.
B2 noun /ˈaʊt.saɪ.dər/

Out-sider

Meaning
a person who does not belong to a particular group; someone not accepted by a group
Example
As an outsider, he found it difficult to understand their traditions.
C2 adjective /ˈɔːrnəri/

ornery

Meaning
bad-tempered and combative; stubborn
Example
The ornery old man refused to leave his chair.
C1 adjective əˈblɪv.i.əs

oblivious

Meaning
Being completely unaware of surrounding events.
Example
He was oblivious to the noise around him while reading.
A1 preposition /ɒn/

on

Meaning
physically in contact with and supported by a surface
Example
The book is on the table.
C1 noun /ɒpˈtɒmətri/

optometry

Meaning
The practice or profession of examining the eyes and prescribing corrective lenses.
Example
She studied optometry at the university to become an eye specialist.
C1 noun /ˈɑːdəsi/

odyssey

Meaning
a long, adventurous journey; a series of experiences
Example
Her career has been an odyssey of challenges and achievements.
A1 interjection /oʊ/

oh

Meaning
used to express surprise, realization, or understanding
Example
Oh, I didn't know you were here!
B2 noun /ˈaʊtɪŋ/

outing

Meaning
a short trip or excursion, often for leisure or enjoyment
Example
They had a lovely outing at the beach last weekend.
A2 adjective/adverb /ˈɒnˌlaɪn/

online

Meaning
connected to or available through the internet
Example
I prefer shopping online rather than going to stores.
C2 adjective /ˌɔːfˈkiː/

off-key

Meaning
out of tune; not in accordance with expected standards
Example
The singer was slightly off-key during the performance.
C1 noun /əˈfen.dər/

Offender

Meaning
a person who commits an illegal act; someone who breaks the law
Example
The young offender was sentenced to community service.
A1 adjective /oʊld/

old

Meaning
having lived for a long time; not new or young
Example
He still drives his old car.
C1 adjective /ˌaʊtˈlændɪʃ/

outlandish

Meaning
Looking or sounding bizarre or unfamiliar; strange.
Example
He wore an outlandish costume to the festival.
B2 verb /əˈpoʊz/

oppose

Meaning
to disagree with or resist; to act against
Example
Many citizens oppose the new tax policy.
C2 noun /ˈoʊdiəm/

odium

Meaning
general or widespread hatred or disgust directed toward someone as a result of their actions
Example
The scandal brought public odium on the corrupt minister.
C1 verb /əˈprɛs/

oppress

Meaning
to keep someone down by harsh and unfair use of authority; to suppress
Example
The dictator oppressed the people for decades.
C1 noun /ˈəʊ.nəs/

onus

Meaning
A duty or responsibility, especially a heavy one.
Example
The onus is on the government to provide relief to the victims.
C1 adjective /əbˈdʒek.ʃən.ə.bəl/

Objectionable

Meaning
arousing distaste or opposition; unpleasant or offensive
Example
His objectionable behavior made everyone uncomfortable.
C1 noun /ˈaʊtriːtʃ/

outreach

Meaning
The act of providing services or support to people who might not otherwise have access to them.
Example
The charity launched a new outreach program for the homeless.
C2 verb /ˈɑːskjʊleɪt/

osculate

Meaning
To kiss.
Example
They osculated under the mistletoe during the party.
C1 verb /ˈɒp.tɪ.maɪz/

optimize

Meaning
To make the best or most effective use of a situation, opportunity, or resource.
Example
The company is working to optimize its supply chain.
C1 adjective /ˈɔːrθədɑːks/

orthodox

Meaning
conforming to traditional beliefs or practices
Example
He comes from a very orthodox family.
C2 noun /ˈɒrɪz(ə)n/

orison

Meaning
a prayer, especially a devout or formal one
Example
He whispered a quiet orison before going to sleep.
A1 pronoun /aʊərz/

ours

Meaning
belonging to or associated with us
Example
This victory is ours.
C1 noun /ˈɔːr.ə.tɔːr.i/

oratory

Meaning
The art of public speaking; skilled and confident delivery of speeches to an audience.
Example
His exceptional oratory skills won him the debate competition.