oxygenation
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C1 noun /ˌɒk.sɪ.dʒɪˈneɪ.ʃən/

oxygenation

Meaning
The process of supplying or treating with oxygen.
Example
Deep breathing exercises improve oxygenation of the body.
C1 verb /aʊtˈlɪv/

outlive

Meaning
to live longer than someone or something else
Example
She outlived all her siblings.
C2 verb /ˌoʊvərˈliːp/

overleap

Meaning
To jump over something; to omit or disregard something.
Example
He managed to overleap the fence easily.
B1 noun /ˈɔːfɪsər/

officer

Meaning
a person in a position of authority, especially in law enforcement
Example
The officer gave him a ticket for speeding.
C1 verb /ˌoʊvərˈʃædoʊ/

overshadow

Meaning
to appear more important or significant than something else, reducing its impact
Example
The scandal overshadowed his achievements.
C1 adjective /ˌɒstɛnˈteɪʃəs/

ostentatious

Meaning
Characterized by a showy or pretentious display intended to impress others.
Example
He wore an ostentatious gold chain to the party.
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.
A2 preposition /ˈɒntu/

onto

Meaning
moving to a position on the surface of something
Example
She jumped onto the table to get the book.
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.
B2 noun, verb /ˈaʊtlɔː/

outlaw

Meaning
as a noun: a person who breaks the law and lives outside its protection; as a verb: to make something illegal
Example
The government decided to outlaw the harmful practice.
C1 verb /ˌoʊvərˈriːtʃ/

overreach

Meaning
to go beyond what is proper, acceptable, or achievable; to overextend
Example
The politician overreached by making promises he couldn't keep.
C1 adjective /ˌɔːrnəˈmɛntl/

ornamental

Meaning
Serving as decoration; decorative rather than functional.
Example
The garden was filled with ornamental plants.
C1 noun /əˈpreʃ.ən/

Oppression

Meaning
cruel or unjust treatment or exercise of authority; the state of being subject to harsh treatment
Example
The people fought against political oppression.
B2 verb /əˈblaɪdʒ/

oblige

Meaning
to compel or require someone to do something; to help or do a favor
Example
I would be happy to oblige you with the information you need.
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.
A2 adverb /ˈɒbviəsli/

obviously

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

orgy

Meaning
A wild party involving excessive indulgence, especially of a sexual nature.
Example
The novel described a scandalous orgy in a wealthy mansion.
B2 verb /əˈfend/

offend

Meaning
to cause displeasure or resentment; to hurt someone's feelings
Example
His rude comments offended many people at the meeting.
B2 noun /ˈaʊt.breɪk/

Out-break

Meaning
a sudden occurrence or eruption of something, especially disease or violence
Example
The outbreak of flu affected many students in the school.
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.
C1 noun /ˈɒstrəsɪzəm/

ostracism

Meaning
The act of excluding someone from a group or society.
Example
The student faced ostracism after breaking the rules.
C1 noun ɔːrˈdiːl

ordeal

Meaning
A physical or mental painful experience that must be endured
Example
Surviving the wilderness was a tough ordeal for them.
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.
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.
C1 noun /ˈɑːdəsi/

odyssey

Meaning
a long, adventurous journey; a series of experiences
Example
Her career has been an odyssey of challenges and achievements.
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 /ˈɔː.də.li/

Orderly

Meaning
well-organized; systematic; peaceful
Example
Please keep your desk orderly and clean.
C1 adverb /ˈɒp.tɪ.məl.i/

optimally

Meaning
In the most effective or efficient way possible.
Example
Tasks should be allocated optimally for maximum output.
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.
B1 noun ˌɒpəˈtjuːnɪtiz

opportunities

Meaning
A set of circumstances that makes it possible to do something; chances for advancement or success.
Example
Opportunities for jobs decreased.
C1 verb əˈblɪt.ə.reɪt

obliterate

Meaning
To destroy utterly; wipe out completely, to erase or remove something completely.
Example
The storm obliterated entire villages.
C2 adjective /ˌoʊpəˈlɛsənt/

opalescent

Meaning
showing or reflecting a milky, iridescent light like an opal
Example
The opalescent sky glowed with shades of pink and blue.
B1 noun /ˈɑː.lɪv/

Olive

Meaning
a small oval fruit with a hard pit, typically green or black, used for eating or making oil
Example
Olive oil is considered one of the healthiest cooking oils available.
B1 noun əbˈdʒek.tɪv

objective

Meaning
A thing aimed at or sought; a goal.
Example
The main objective is to test sustainability.
B2 adverb /əˈfɪʃəli/

officially

Meaning
in a formal or authorized way
Example
The building was officially opened by the mayor.
B1 noun /ˌɒpəˈreɪʃən/

operation

Meaning
an act or process of functioning or operating
Example
The hospital performed a successful operation.
C2 noun /ˈɒlɪv bɑːb/

Olive barb

Meaning
A small freshwater fish with olive-colored body and barbels
Example
The olive barb is commonly found in South Asian rivers.
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.
C1 adjective /əbˈdʒek.ʃən.ə.bəl/

Objectionable

Meaning
arousing distaste or opposition; unpleasant or offensive
Example
His objectionable behavior made everyone uncomfortable.
B2 noun /əbˈsɛʃən/

obsession

Meaning
An idea, thought, or concern that continually preoccupies or intrudes on a person's mind.
Example
His obsession with success drove him to work day and night.
B1 noun /əˈkeɪ.ʒən/

Occasion

Meaning
a particular time or instance of an event; a special celebration
Example
Their wedding was a joyous occasion.
C1 adjective /əbˈstrʌk.tɪv/

Obstructive

Meaning
deliberately creating difficulties or preventing progress; hindering or blocking something
Example
The obstructive policies hindered economic growth.
C2 noun /ɒkˈteɪvoʊ/

octavo

Meaning
A size of book page resulting from folding a sheet of paper into eight leaves (sixteen pages).
Example
The old library contained rare octavo editions of Shakespeare.
C2 adjective /əbˈtruːsɪv/

obtrusive

Meaning
noticeable in an unpleasant or unwelcome way; intrusive
Example
The advertisement was so obtrusive that it ruined the user’s experience.
C1 adjective /ˌaʊtˈlændɪʃ/

outlandish

Meaning
Looking or sounding bizarre or unfamiliar; strange.
Example
He wore an outlandish costume to the festival.
C1 noun /ˈɑːlɪˌɡɑːrki/

oligarchy

Meaning
A small group of people having control of a country, organization, or institution.
Example
The nation was ruled by a wealthy oligarchy that controlled all resources.
C1 noun /əˈkɜr.əns/

Occurrence

Meaning
an instance or event of something happening; the fact of occurring
Example
The occurrence of earthquakes is common in this region.
B2 noun /ˌaʊtˈsaɪdə(r)/

outsider

Meaning
a person who is not accepted as a member of a particular group or community
Example
As a new student, she often felt like an outsider at school.
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.
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 verb /ˈɒs.ɪ.leɪt/

oscillate

Meaning
To swing or move back and forth between two points or states
Example
The stock market tends to oscillate during uncertain times.
C2 noun /ˈəʊpəs/

opus

Meaning
A creative work, especially a large and important piece of music or art.
Example
The composer considered the symphony his greatest opus.
B2 noun /ˈoʊvərpæs/

overpass

Meaning
A bridge that carries one road or railway over another.
Example
The new overpass reduced traffic congestion in the city.
A1 pronoun /aʊər/

our

Meaning
belonging to or associated with the speaker and one or more other people
Example
This is our house.
C2 noun ˈɒpjʊləns

opulence

Meaning
Great wealth or luxuriousness.
Example
The opulence of the mansion was evident in its ornate furnishings.
B2 adverb /ˈaʊtraɪt/

Outright

Meaning
completely; directly; without reservation or concealment
Example
She rejected the proposal outright.
C1 adjective/adverb /ˌɒfˈhænd/

offhand

Meaning
Without previous thought or preparation; casual and sometimes dismissive.
Example
She gave an offhand reply to his serious question.
C1 noun /ɔːr/

Oar

Meaning
a long pole with a flat blade used to row a boat
Example
He dipped the oar into the water and pulled the boat forward.
B2 adverb /ˈɒdli/

oddly

Meaning
in a strange or unusual way; unexpectedly
Example
Oddly, he didn’t seem upset by the bad news.
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 /əbˈdʒɛktər/

objector

Meaning
a person who expresses opposition or disapproval to something
Example
The objector raised his concerns during the town meeting.
B2 noun /ˌɑb.zɚˈveɪ.ʃən/

Observation

Meaning
the action of watching something carefully; a comment or remark
Example
The scientist made careful observations of the experiment.
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 /ɑːks/

Ox

Meaning
a large strong animal used for pulling heavy loads or plowing fields
Example
The farmer used an ox to plow the field.
C2 adjective /ɒmˈnɪp.ə.tənt/

omnipotent

Meaning
Having unlimited power; able to do anything; all-powerful.
Example
In many religions, God is considered omnipotent.
C1 noun /əbˈzɜːrvətɔːri/

observatory

Meaning
A building equipped for observing astronomical or meteorological events.
Example
The observatory is open to the public on weekends.
C2 adjective /ɑːlˈfæktəri/

olfactory

Meaning
Relating to the sense of smell.
Example
The perfume activated her olfactory nerves immediately.
C1 noun /ˈɑd.ɪ.ti/

Oddity

Meaning
something that is strange or unusual; a peculiar trait
Example
His habit of sleeping standing up was considered an oddity.
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.
B2 adjective /ˈɔɪl.i/

Oily

Meaning
covered with oil; containing oil; greasy
Example
Her oily skin needs special care.
C1 noun ˈaʊt.sɔːr.sɪŋ

outsourcing

Meaning
The practice of having certain job functions done outside a company instead of having an in-house department or employee handle them.
Example
Many companies use outsourcing to reduce labor costs.
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.
B2 noun ˈəʊnəʃɪp

ownership

Meaning
The act, state, or right of possessing something; legal title to something.
Example
Ownership of resources was crucial.
B2 noun /oʊk/

oak

Meaning
a large tree that produces acorns and has strong, hard wood
Example
The old oak tree provided shade for the entire yard.
B2 adverb /ˈoʊpənli/

Openly

Meaning
in a frank and honest way; without concealment; publicly
Example
She spoke openly about her struggles with mental health.
C2 adjective /ˈoʊ.kən/

oaken

Meaning
made of or resembling oak wood
Example
The villagers gathered around the oaken table in the hall.
B2 noun /ˈɒp.tɪ.mɪ.zəm/

optimism

Meaning
Hopefulness and confidence about the future or the successful outcome of something.
Example
Her optimism helped her overcome difficult times.
C1 adjective /ˌəʊ.vərˈɑː.tʃɪŋ/

overarching

Meaning
Comprehensive or all-embracing; forming an arch over something.
Example
The overarching theme of the book is human resilience.
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.
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.
C2 noun /ˈɒn.rʌʃ/

onrush

Meaning
A sudden powerful forward or downward rush or flow.
Example
The onrush of water swept away the bridge.
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.
B2 adjective /ˈɒp.ʃən.əl/

Optional

Meaning
not compulsory; left to choice
Example
Wearing a tie is optional for this event.
C1 noun ˌɒb.səˈlɛs.əns

obsolescence

Meaning
The process of becoming obsolete or outdated and no longer used.
Example
The rapid advancement of technology leads to the obsolescence of old devices.
B2 verb /ˌoʊvərˈtɜrn/

overturn

Meaning
to turn something upside down or to reverse a decision or judgment
Example
The court decided to overturn the previous ruling.
C1 adjective /əˈkʌlt/

occult

Meaning
Relating to mysterious, supernatural, or magical powers and practices.
Example
She had always been fascinated by the occult and its hidden knowledge.
C1 adjective /ˌɒmnɪˈprezənt/

omnipresent

Meaning
(of God) present everywhere at the same time.
Example
Social media has become an omnipresent force in modern society.
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.
B2 noun /ɒpˈtɪʃən/

optician

Meaning
A person who makes or sells glasses and contact lenses.
Example
The optician adjusted my new glasses to fit better.
B2 noun /ˈɔːɡən/

organ

Meaning
a part of the body that performs a specific function
Example
The heart is an important organ in the human body.
C2 noun /ˈɔːrdnəns/

ordnance

Meaning
Military supplies including weapons, ammunition, and combat equipment.
Example
The army stored ordnance in a secure facility outside the city.
C1 noun /ˌɑːpərˈtuːnɪst/

opportunist

Meaning
A person who takes advantage of situations for personal gain, often disregarding principles.
Example
He is seen as an opportunist who always puts his interests first.
C2 noun /əˈprəʊ.bri.əm/

opprobrium

Meaning
Harsh criticism or public disgrace.
Example
The scandal brought opprobrium on the politician.
C2 noun /ˌɒktədʒəˈnɛəriən/

octogenarian

Meaning
A person who is between 80 and 89 years old.
Example
The octogenarian still jogs every morning to stay healthy.
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.
B2 verb /ˌoʊvərˈpeɪ/

overpay

Meaning
To pay more than is necessary or reasonable.
Example
She realized she had overpaid for the concert tickets.
C2 adjective /ˈɒsiəs/

osseous

Meaning
consisting of or turned into bone; bony
Example
The fossil showed osseous structures of the ancient creature.
C1 verb /ˌaʊtˈstrɛtʃ/

outstretch

Meaning
To extend or stretch something outward.
Example
He outstretched his hand to help her up.
C1 adjective /ˌaʊt ən ˈaʊt/

out-and-out

Meaning
complete; absolute; used to emphasize something extreme
Example
The speech was an out-and-out lie.
C2 verb /əbˈtruːd/

obtrude

Meaning
to force oneself or one's ideas into a situation uninvited or inappropriately
Example
He didn’t want to obtrude into their private conversation.
C1 adjective /əˈblaɪdʒɪŋ/

obliging

Meaning
Willing to help or do favors; accommodating.
Example
The staff at the hotel were very obliging and friendly.
C1 noun /ˈaʊtˌkraɪ/

outcry

Meaning
A strong expression of public anger or disapproval.
Example
The decision sparked an outcry among the citizens.
B2 noun /ɒdz/

odds

Meaning
the likelihood or probability of something happening; chances for or against something
Example
The odds of winning the lottery are very low.
C1 noun /ˈɒn.slɔːt/

onslaught

Meaning
A fierce or destructive attack.
Example
The village faced an onslaught of enemy forces.
C2 noun /ˈɒlɪv brɑːntʃ/

olive-branch

Meaning
A symbol of peace or reconciliation; an offer to end conflict.
Example
He extended an olive-branch to his rival after years of hostility.
B2 noun /ˈaʊtˌskɜːrt/

outskirt

Meaning
the outer edge or border of a town or city
Example
They built a new factory on the outskirts of the city.
A1 adjective/adverb /ˈoʊnli/

only

Meaning
without others; solely
Example
He is the only person who knows the secret.
C1 adjective /ˈɔːrθədɑːks/

orthodox

Meaning
conforming to traditional beliefs or practices
Example
He comes from a very orthodox family.
C2 noun /əbˈsɪdiən/

obsidian

Meaning
A hard, dark, glasslike volcanic rock formed by rapid solidification of lava.
Example
The knife was made from sharp obsidian.
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.
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.
B1 noun ˌɔː.ɡən.aɪˈzeɪ.ʃənz

organizations

Meaning
Organized groups of people with a particular purpose
Example
Organizations play a key role in policymaking.
C2 noun /ˈɔːrɪndʒfɪn ˈleɪbiːoʊ/

Orangefin labeo

Meaning
a freshwater fish species native to South Asian rivers, commonly found in Bangladesh and India
Example
The orangefin labeo is an important food fish in Bangladesh.
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.
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.
C2 adjective /ˌɒbsəˈlɛsnt/

obsolescent

Meaning
Becoming outdated or on the way to becoming obsolete.
Example
The obsolescent technology could not compete with modern devices.
C1 noun /ˈɒfʃuːt/

offshoot

Meaning
A branch or result that develops from something larger or more important.
Example
The startup was an offshoot of a major tech company.
C1 adjective əˈblɪv.i.əs

oblivious

Meaning
Being completely unaware of surrounding events.
Example
He was oblivious to the noise around him while reading.
B1 verb /ˈɔːrɡənaɪz/

organize

Meaning
to arrange systematically; to plan and coordinate
Example
She organized a surprise party for her friend.