ocean acidification
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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.
C2 adjective /ɒmˈnɪʃ.ənt/

Omniscient

Meaning
having complete knowledge; all-knowing
Example
The narrator in the novel appears to be omniscient, knowing every character's thoughts.
C2 noun /ˈɒbsɪkwiːz/

obsequies

Meaning
Funeral rites or ceremonies.
Example
The obsequies for the late leader drew thousands of mourners.
C1 verb /ˌoʊvərˈruːl/

overrule

Meaning
to reject or disallow by exercising one's superior authority; to override a decision
Example
The judge overruled the objection from the defense attorney.
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.
C2 noun (plural) /ˈɔːrdʒiːz/

orgies

Meaning
Wild gatherings involving excessive indulgence in food, drink, or sexual activity.
Example
The ancient Romans were infamous for their lavish feasts and orgies.
B1 adjective /əˈfɪʃəl/

official

Meaning
relating to or connected with an authority or public office
Example
He is the official spokesperson for the company.
B2 verb /ˈɒkjʊpaɪ/

occupy

Meaning
to take up space or time; to live or work in a place
Example
The new family will occupy the house next month.
A1 adverb /ɔːf/

off

Meaning
away from a place, position, or time; not functioning
Example
He turned off the light before leaving the room.
C2 adjective /əbˈsiːkwɪəs/

obsequious

Meaning
Excessively obedient or attentive in a servile way.
Example
The waiter gave an obsequious bow to every customer.
B2 noun ˈɔː.bɪt

orbit

Meaning
The curved path of a celestial object around a star, planet, or moon.
Example
The satellite maintains its orbit around Earth.
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.
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.
C1 verb /aʊtˈlɪv/

outlive

Meaning
to live longer than someone or something else
Example
She outlived all her siblings.
B1 verb অকার্স

occurs

Meaning
Happens or takes place, especially as a result of something.
Example
Snowfall occurs in regions with freezing temperatures.
C1 verb /ˈɔːstrəˌsaɪz/

ostracize

Meaning
to exclude someone from a group or society; to banish or shun
Example
After the scandal, he was ostracized by his former colleagues.
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.
B2 noun /ˈɒt.ər/

Otter

Meaning
a mammal with webbed feet that lives near water and feeds on fish
Example
The playful otter swam gracefully in the river.
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.
A2 noun /ˈoʊ.ʃən/

ocean

Meaning
A very large expanse of sea, especially the main areas of sea between continents.
Example
The ocean covers more than 70% of the Earth's surface.
B1 noun /ˌɒp.əˈtjuː.nə.ti/

opportunity

Meaning
A favorable chance or set of circumstances that makes it possible to do something.
Example
She seized the opportunity to study abroad.
B2 noun /wʌn θɜːd/

one-third

Meaning
one part of something that is divided into three equal parts
Example
One-third of the class was absent today.
A1 conjunction /ɔːr/

or

Meaning
used to link alternatives
Example
You can have tea or coffee.
C2 noun /ˈɒkjʊlɪst/

oculist

Meaning
An old-fashioned term for an eye doctor or ophthalmologist.
Example
The village oculist was known for treating various eye diseases.
B2 verb əbˈzɜːrvd

observed

Meaning
To celebrate or keep a religious or other festival.
Example
The day is observed annually to raise awareness.
A2 adjective /ˈɑp.ə.zɪt/

Opposite

Meaning
Contrary; completely different; facing each other; reverse;
Example
They have opposite views on politics but remain good friends.
C2 noun /ˈɒbsɪkwi/

obsequy

Meaning
A funeral rite, usually used in plural form obsequies.
Example
The village gathered for the obsequy of their beloved teacher.
B2 noun /ɔːrˈɡæn.ɪk ˈfɑːr.mɪŋ/

organic farming

Meaning
Farming method that avoids synthetic chemicals and fertilizers, using natural methods instead.
Example
Organic farming improves soil health and reduces pollution.
B2 adjective /ˌɔːɡənaɪˈzeɪʃənəl/

organizational

Meaning
relating to the structure or arrangement of a company or group
Example
He works on improving the organizational structure of the company.
C2 noun /ˈɑb.lə.kwi/

Obloquy

Meaning
harsh criticism or verbal abuse; strong public condemnation
Example
The politician faced obloquy after the scandal.
B2 verb /əˈpoʊz/

oppose

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

olfactory

Meaning
Relating to the sense of smell.
Example
The perfume activated her olfactory nerves immediately.
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.
B2 noun /ɔːˈræŋ.ʊˌtæŋ/

Orang-Outang

Meaning
a large arboreal ape with long reddish hair, long arms, and hooked hands and feet, native to Borneo and Sumatra
Example
The orangutan swings gracefully through the forest canopy.
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 /ˈɔːɡənɪzəm/

organism

Meaning
a living thing that has an organized structure and can grow and reproduce
Example
Every organism needs water to survive.
B2 noun /əbˈzɜːvər/

observer

Meaning
a person who watches or notices something
Example
The observer took notes during the meeting.
C1 adjective /oʊˈvɜːrt/

overt

Meaning
Done or shown openly; not hidden or secret.
Example
There was an overt display of hostility between the rivals.
B1 noun /ˈɒbstəkəlz/

obstacles

Meaning
Things that block one's way or prevent or hinder progress.
Example
Obstacles in life can often lead to valuable lessons.
C1 adjective /ˈəʊnərəs/

onerous

Meaning
involving a great deal of effort, difficulty, or burden
Example
Fulfilling the contract proved to be an onerous task for the company.
B2 noun /ˌɔː.ri.enˈteɪ.ʃən/

orientation

Meaning
the process of becoming familiar with a new situation or environment
Example
The company provides an orientation for all new employees.
C2 noun ˈoʊʃən ˌstrætɪfɪˈkeɪʃən

ocean stratification

Meaning
The formation of layers in ocean water due to differences in density.
Example
Ocean stratification affects marine biodiversity.
C1 adjective ˈɒb.stɪ.nət

obstinate

Meaning
Refusing to change one's mind or accept others' advice easily.
Example
He remained obstinate despite the logical arguments.
C2 adjective /ˈoʊdiəs/

odious

Meaning
extremely unpleasant or repulsive
Example
The dictator was remembered for his odious crimes.
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.
C1 noun /ˌɒbstəˈtrɪʃən/

obstetrician

Meaning
A doctor specializing in pregnancy, childbirth, and women's reproductive health.
Example
The obstetrician monitored the mother’s health throughout the pregnancy.
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/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 /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 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 /ˈoʊvərˌtʊr/

overture

Meaning
An introduction to something more substantial; a proposal or preliminary action.
Example
The peace overture was welcomed by both nations.
C2 adjective /əˈrækjələr/

oracular

Meaning
Resembling an oracle; giving wise, mysterious, or prophetic statements
Example
Her oracular words left the audience deep in thought.
B2 verb /ˌoʊvərˈloʊd/

overload

Meaning
to load with too much weight or cargo; to give too much work or information
Example
Don't overload the washing machine or it might break down.
B2 noun /oʊˈbiː.di.əns/

Obedience

Meaning
compliance with an order, request, or law; submission to authority
Example
Children should show obedience to their parents.
C2 noun /ˈɔː.rɪ.fɪs/

orifice

Meaning
an opening or hole, especially in the body or a device
Example
The medicine was applied through a small orifice in the device.
B2 noun /əˈfɛns/

offense

Meaning
an illegal act; a crime
Example
The offense was committed in broad daylight.
C1 adjective /əbˈtjuːs/

Obtuse

Meaning
annoyingly insensitive or slow to understand; lacking in intelligence or perception
Example
He was being deliberately obtuse about the instructions.
C1 adjective /ˌɔːrθəˈpiːdɪk/

orthopedic

Meaning
Relating to the correction of deformities of bones or muscles.
Example
She visited an orthopedic surgeon for her back pain.
A2 adjective/adverb /ˈɒnˌlaɪn/

online

Meaning
connected to or available through the internet
Example
I prefer shopping online rather than going to stores.
C1 noun /ˈoʊvərˌsiːər/

overseer

Meaning
a person who supervises work or workers; a manager
Example
The overseer made sure the workers followed safety rules.
C1 adjective /ˈɑp.ər.ə.tɪv/

Operative

Meaning
Functioning; in effect; working; active;
Example
The new safety protocols became operative immediately after the accident.
C2 noun /ˈɔːrzmən/

Oarsman

Meaning
a person who rows a boat, especially as a member of a racing crew
Example
The experienced oarsman rowed the boat across the river.
B2 noun /ˈaʊtbɜːrst/

outburst

Meaning
a sudden release of strong emotion or activity
Example
She had an angry outburst during the meeting.
B2 noun /ˈɔɪ.stər/

Oyster

Meaning
a type of shellfish that can be eaten and sometimes contains pearls
Example
She ordered fresh oysters at the seafood restaurant.
C2 adjective /ˌɒbsəˈlɛsnt/

obsolescent

Meaning
Becoming outdated or on the way to becoming obsolete.
Example
The obsolescent technology could not compete with modern devices.
B2 noun /əˌrɪʒ.əˈnæl.ə.ti/

Originality

Meaning
the quality of being novel or unusual; the ability to think independently and creatively
Example
The artist's work showed great originality and creativity.
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 /ˈaʊtˌkraɪ/

outcry

Meaning
A strong expression of public anger or disapproval.
Example
The decision sparked an outcry among the citizens.
A2 adverb; noun /ˌaʊtˈdɔːrz/

outdoors

Meaning
outside a building; the natural environment outside
Example
We spent the whole afternoon outdoors enjoying the fresh air.
B2 noun ˈaʊt.pʊt

output

Meaning
The amount of something produced by a person, machine, or industry.
Example
Climate change reduces agricultural output.
C2 adjective /ˈoʊ.pən ˈhɑrt.ɪd/

Open-hearted

Meaning
Kind and generous; sincere and honest; warmly receptive;
Example
She welcomed the strangers with an open-hearted smile and invited them for dinner.
A2 noun /ˈɔr.dər/

Order

Meaning
a command or instruction; the arrangement of things following a particular sequence
Example
The teacher gave an order to sit down quietly.
B1 adjective /ˈaʊtər/

outer

Meaning
situated on or toward the outside; external
Example
The outer layer of the jacket is waterproof.
C2 verb /ˈɒsɪfaɪ/

ossify

Meaning
to turn into bone or become rigid and inflexible
Example
With age, some tissues tend to ossify.
C1 noun /ˈɒptɪks/

optics

Meaning
The branch of physics dealing with light and vision.
Example
She studied optics to understand how lenses work.
C1 adjective /ˌɒmnɪˈprezənt/

omnipresent

Meaning
(of God) present everywhere at the same time.
Example
Social media has become an omnipresent force in modern society.
C2 noun /ˈaʊtˌrɪɡər/

outrigger

Meaning
a projecting structure on a boat, used for stability, or a boat fitted with such support
Example
The fisherman paddled his outrigger canoe across the lagoon.
C1 noun /oʊˈveɪʃən/

ovation

Meaning
an enthusiastic show of approval, especially by clapping hands
Example
The actor received a standing ovation after his performance.
C1 verb /ˈɒfsɛt/

offset

Meaning
To counterbalance or compensate for something.
Example
The company's losses were offset by gains in other departments.
C2 adjective /ˈoʊ.vɔɪd/

ovoid

Meaning
egg-shaped; resembling an oval or ellipse
Example
The artist painted an ovoid shape in the center of the canvas.
B2 noun /ˈaʊtˌsɛt/

outset

Meaning
the beginning or start of something
Example
From the outset, the project seemed ambitious but achievable.
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.
C1 adjective /əbˈsɛsɪv/

obsessive

Meaning
Relating to or characterized by an obsession; thinking about something too much.
Example
He has an obsessive interest in cleanliness.
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.
C2 noun /əbˈstrʌkʃənɪst/

obstructionist

Meaning
a person who deliberately delays or prevents progress by being uncooperative or oppositional
Example
The obstructionist in the committee kept delaying the decision-making process.
C1 noun /ˌɑːsɪˈleɪʃən/

oscillation

Meaning
The movement back and forth at a regular speed.
Example
The oscillation of the pendulum controls the clock.
B2 adverb /ˌoʊvərˈnaɪt/

Overnight

Meaning
during the night; suddenly; very quickly
Example
The city was transformed overnight.
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.
C2 noun /ˌɔːrnɪˈθɑːlədʒi/

ornithology

Meaning
the scientific study of birds
Example
She pursued ornithology to understand the migration patterns of birds.
B2 adjective ɔːrˈɡæn.ɪk

organic

Meaning
Produced or involving production without the use of chemical fertilizers, pesticides, or other artificial agents.
Example
Organic farming improves soil health and reduces pollution.
C2 noun /ˈoʊ.vər.let/

Overlet

Meaning
a thin mattress or pad placed over a bed for additional comfort
Example
The overlet makes the bed more comfortable to sleep on.
B2 noun /ˈɔːrnəmənt/

ornament

Meaning
A decorative object that adds beauty to something.
Example
The Christmas tree was decorated with colorful ornaments.
A1 preposition /ɒn/

on

Meaning
physically in contact with and supported by a surface
Example
The book is on the table.
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 verb /ˈoʊɡəl/

ogle

Meaning
To look at someone with obvious sexual interest.
Example
He kept ogling the models on the runway.
C1 adjective /əbˈzɜːrvənt/

Observant

Meaning
quick to notice things; watchful; attentive
Example
The observant detective noticed every small detail at the crime scene.
C1 adjective /ɒˈstɛnsəbl/

ostensible

Meaning
stated or appearing to be true, but not necessarily so
Example
His ostensible reason for leaving was illness, but many doubted it.
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 /əˈfend/

offend

Meaning
to cause displeasure or resentment; to hurt someone's feelings
Example
His rude comments offended many people at the meeting.
B2 adjective /ˈɔː.də.li/

Orderly

Meaning
well-organized; systematic; peaceful
Example
Please keep your desk orderly and clean.
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.
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.
C2 noun /ˈɒksɪdənt/

occident

Meaning
The countries of the West, especially Europe and America.
Example
The Occident has a long history of philosophical thought.
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.
A2 noun /ˈɒpərə/

opera

Meaning
a dramatic performance in which the actors sing most of their lines
Example
We watched a beautiful opera at the theater last night.
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.
C1 adjective /ɔːˈneɪt/

ornate

Meaning
elaborately or highly decorated
Example
The palace hall was filled with ornate furniture and paintings.
B2 adjective /əˈpəʊzd/

opposed

Meaning
being against or in disagreement with something
Example
She is strongly opposed to animal testing.
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.
C1 noun /oʊˈeɪ.sɪs/

Oasis

Meaning
a fertile spot in a desert where water is found; a pleasant refuge
Example
The travelers found an oasis after days in the desert.
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 noun /ˌɔːrdɪˈneɪʃən/

ordination

Meaning
The ceremony of conferring holy orders or official religious authority.
Example
The church held a special service for the ordination of new priests.
C1 verb ˈɔːr.kɪ.streɪt

orchestrate

Meaning
To plan or coordinate the elements of (a situation) to produce a desired effect, especially surreptitiously.
Example
He skillfully orchestrated the entire event.
C1 noun /əbˈdʒɛktər/

objector

Meaning
a person who expresses opposition or disapproval to something
Example
The objector raised his concerns during the town meeting.
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.
C2 noun /ɔːˈreɪʃən/

oration

Meaning
A formal speech, especially one given on a ceremonial occasion
Example
The president delivered an inspiring oration.
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 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.
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.
C2 noun /ɔːrˈθɑːɡrəfi/

orthography

Meaning
The conventional spelling system of a language.
Example
English orthography can be confusing for learners.
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.