overcrowding
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B2 noun /ˌoʊ.vɚˈkraʊ.dɪŋ/

overcrowding

Meaning
Excessive crowd or packed condition, having more people or vehicles in a place than the approved capacity.
Example
The subway system suffers from severe overcrowding during peak hours.
B1 noun /əˈkeɪ.ʒən/

Occasion

Meaning
a particular time or instance of an event; a special celebration
Example
Their wedding was a joyous occasion.
C2 verb /ˈɑːblɪˌɡeɪt/

obligate

Meaning
to bind legally or morally; to compel someone to do something
Example
The contract obligates the company to provide timely service.
B2 adjective /ˌoʊvərˈwɛlmɪŋ/

overwhelming

Meaning
Very intense; overpowering.
Example
Large tasks can feel overwhelming.
B2 noun /ˈaʊtˌfɪt/

outfit

Meaning
A set of clothes worn together, often for a particular occasion.
Example
She wore a beautiful outfit to the party.
C1 verb /aʊtˈlæst/

outlast

Meaning
to continue longer than; to endure beyond someone or something else
Example
The old castle has outlasted many generations.
B2 noun ˈoʊzoʊn dɪˈpliʃən

ozone depletion

Meaning
The reduction in the amount of ozone in the Earth's stratosphere, leading to increased UV radiation.
Example
Ozone depletion increases UV radiation.
B2 noun ˈaʊt.pʊt

output

Meaning
The amount of something produced by a person, machine, or industry.
Example
Climate change reduces agricultural output.
B2 noun /ˈoʊ.vər ɡroʊθ/

Over growth

Meaning
excessive growth beyond normal size; too much development or expansion
Example
The overgrowth of weeds made the garden look untidy.
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.
C1 noun /ˌoʊvərprəˈdʌkʃən/

overproduction

Meaning
The act of producing more of something than is needed or can be consumed.
Example
The economic crisis was caused by the overproduction of goods.
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.
B2 verb /əˈrɪdʒɪneɪt/

originate

Meaning
to come into existence; to begin; to create or produce
Example
The tradition originated in ancient times.
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 /əbˈsiːkwɪəs/

obsequious

Meaning
Excessively obedient or attentive in a servile way.
Example
The waiter gave an obsequious bow to every customer.
C1 adjective /ˌɔːrnəˈmɛntl/

ornamental

Meaning
Serving as decoration; decorative rather than functional.
Example
The garden was filled with ornamental plants.
B2 noun /ˈaʊt.laɪn/

Outline

Meaning
a general description or plan showing the essential features of something; the main points
Example
The teacher provided an outline of the course syllabus.
C1 noun /oʊθ/

Oath

Meaning
a solemn promise or declaration; a sworn statement
Example
The doctor took an oath to help patients.
A2 modal verb /ɔːt/

ought

Meaning
Used to indicate duty, correctness, or moral obligation.
Example
You ought to respect your elders.
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.
C2 noun ˌɒbfəsˈkeɪʃən

obfuscation

Meaning
The action of making something obscure, unclear, or unintelligible.
Example
The obfuscation of the data made it difficult to interpret.
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.
B2 adjective /aʊtˈreɪdʒəs/

outrageous

Meaning
Shockingly bad or excessive; very unusual or amusing.
Example
The comedian told an outrageous joke.
B2 adverb /əˈfɪʃəli/

officially

Meaning
in a formal or authorized way
Example
The building was officially opened by the mayor.
B2 adjective /ˌfæʃ.ənd/

Outdated

Meaning
obsolete; no longer current
Example
This computer software is completely outdated.
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 /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.
A1 preposition /ʌv/

of

Meaning
expressing the relationship between a part and a whole
Example
The book is on the table, in front of the window.
A1 adjective /əʊˈkeɪ/

okay

Meaning
used to express agreement or acceptance
Example
It's okay to leave early today.
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 /oʊˈmɪt/

omit

Meaning
to leave out or exclude; to fail to include something
Example
Please don't omit any important details from your report.
B1 adjective /əˈfɪʃəl/

official

Meaning
relating to or connected with an authority or public office
Example
He is the official spokesperson for the company.
A2 adjective /əˈlɪmpɪk/

olympic

Meaning
related to the Olympic Games, a major international sporting event
Example
She trained hard to compete in the Olympic Games.
C2 verb /ˌaʊtməˈnuːvər/

outmaneuver

Meaning
to gain an advantage over an opponent by skillful or clever action
Example
The smaller company managed to outmaneuver its larger rival.
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.
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.
B1 verb /ˈɒpəreɪt/

operate

Meaning
to function; to control or manage; to perform surgery
Example
The surgeon will operate on the patient tomorrow.
C2 adjective /ɒmˈnɪp.ə.tənt/

omnipotent

Meaning
Having unlimited power; able to do anything; all-powerful.
Example
In many religions, God is considered omnipotent.
A1 adverb /ɔːf/

off

Meaning
away from a place, position, or time; not functioning
Example
He turned off the light before leaving the room.
B2 noun /əˈpoʊ.nənt/

Opponent

Meaning
a person who disagrees with or resists a proposal or practice; someone who competes against another
Example
The chess player defeated his strongest opponent.
C2 noun /ˈoʊɡər/

ogre

Meaning
A mythical monster, often depicted as large, hideous, and cruel; metaphorically, a cruel or terrifying person.
Example
The villagers feared the ogre who lived in the dark forest.
B1 adverb /ˈʌðərwaɪz/

Otherwise

Meaning
in a different way; if not; or else; apart from that
Example
Study hard, otherwise you will fail the exam.
C2 adjective /ˈɑːbdjʊrət/

obdurate

Meaning
Stubbornly refusing to change one's opinion or course of action.
Example
He remained obdurate despite everyone's pleas.
C1 adjective /ˈɑp.ər.ə.tɪv/

Operative

Meaning
Functioning; in effect; working; active;
Example
The new safety protocols became operative immediately after the accident.
C1 verb /ˌoʊvərˈrʌn/

overrun

Meaning
to spread over or occupy completely; to exceed a limit or time
Example
The garden was overrun with weeds.
B2 noun /ˌɒpəˈzɪʃən/

opposition

Meaning
the action of opposing or resisting something
Example
The opposition to the new law was strong.
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.
A1 conjunction /ɔːr/

or

Meaning
used to link alternatives
Example
You can have tea or coffee.
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.
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 verb /aʊtˈwɪt/

outwit

Meaning
to defeat or gain an advantage over someone by being more clever or cunning
Example
The fox managed to outwit the hunters.
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.
C2 verb /əbˈvɜːrt/

obvert

Meaning
To turn something so as to face in a different direction; in logic, to convert a proposition by negating both subject and predicate.
Example
The teacher asked the students to obvert the statement for practice.
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.
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 noun /oʊf/

Oaf

Meaning
a clumsy, stupid, or insensitive person
Example
Don't be such an oaf - handle the dishes carefully!
A2 adverb /ˈɒbviəsli/

obviously

Meaning
in a way that is easily perceived or understood; clearly
Example
Obviously, he didn’t read the instructions.
C1 adjective /ɔːˈneɪt/

ornate

Meaning
elaborately or highly decorated
Example
The palace hall was filled with ornate furniture and paintings.
C2 adjective /ɒˈstɛnsɪv/

ostensive

Meaning
clearly demonstrative or pointing out; explicitly shown
Example
The teacher gave an ostensive example to clarify the concept.
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.
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.
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 /ˈɒksɪdənt/

occident

Meaning
The countries of the West, especially Europe and America.
Example
The Occident has a long history of philosophical thought.
A1 preposition /ɒn/

on

Meaning
physically in contact with and supported by a surface
Example
The book is on the table.
C2 verb ˈɑːb.fə.skeɪt

obfuscate

Meaning
Render obscure, unclear, or unintelligible.
Example
Politicians often obfuscate facts to mislead people.
C2 adjective /ˈoʊdiəs/

odious

Meaning
extremely unpleasant or repulsive
Example
The dictator was remembered for his odious crimes.
A1 adjective/adverb /ˈoʊnli/

only

Meaning
without others; solely
Example
He is the only person who knows the secret.
B1 verb অকার্স

occurs

Meaning
Happens or takes place, especially as a result of something.
Example
Snowfall occurs in regions with freezing temperatures.
B2 adjective ˌɒp.ərˈeɪ.ʃən.əl

operational

Meaning
In working order; ready for use.
Example
The space project will be operational by 2025.
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.
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.
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.
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.
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.
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.
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 adjective /ˈɔːrθədɑːks/

orthodox

Meaning
conforming to traditional beliefs or practices
Example
He comes from a very orthodox family.
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.
C1 adjective /ˌɒbsəˈliːt/

obsolete

Meaning
No longer in use or out of date.
Example
Floppy disks are now completely obsolete.
C1 noun /ˌəʊvərɪksˌplɔɪˈteɪʃən/

overexploitation

Meaning
The unsustainable use of natural resources at a rate that exceeds their ability to regenerate, leading to depletion and environmental degradation.
Example
Overexploitation of natural resources leads to environmental degradation.
C2 adjective /ˈɒsiəs/

osseous

Meaning
consisting of or turned into bone; bony
Example
The fossil showed osseous structures of the ancient creature.
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.
C2 noun /ˈɔɪl.mən/

Oilman

Meaning
a person who works in the oil industry; someone who sells oil or petroleum products
Example
The oilman discovered a new oil field in the desert.
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 /ˈaʊtbɜːrst/

outburst

Meaning
a sudden release of strong emotion or activity
Example
She had an angry outburst during the meeting.
C1 verb /ˈɒfsɛt/

offset

Meaning
To counterbalance or compensate for something.
Example
The company's losses were offset by gains in other departments.
C1 adjective ˈɒptɪmaɪzd

optimized

Meaning
Made as fully perfect, functional, or effective as possible; improved to achieve maximum efficiency.
Example
The system has been optimized for faster performance.
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.
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.
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.
B2 adjective /aʊtˈstændɪŋ/

outstanding

Meaning
extremely good or impressive; not yet resolved or paid
Example
She received an award for her outstanding performance.
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 /oʊˈveɪʃən/

ovation

Meaning
an enthusiastic show of approval, especially by clapping hands
Example
The actor received a standing ovation after his performance.
C2 noun /ˈoʊpiət/

opiate

Meaning
a drug containing opium or its derivatives, used to relieve pain or cause sleep
Example
The doctor prescribed an opiate to relieve the patient's pain.
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 adjective /ˈoʊ.kən/

oaken

Meaning
made of or resembling oak wood
Example
The villagers gathered around the oaken table in the hall.
B2 noun ˈɒksɪdʒən prəˈdʌkʃən

oxygen production

Meaning
The process of generating oxygen through photosynthesis or other means.
Example
Plants are vital for oxygen production and the maintenance of life.
B1 noun /ˌɒpəˈreɪʃən/

operation

Meaning
an act or process of functioning or operating
Example
The hospital performed a successful operation.
B2 verb /ɒpt/

opt

Meaning
to choose or decide to do something instead of something else
Example
Many students opt to study abroad for better opportunities.
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.
B2 verb /ˌoʊvərˈteɪk/

overtake

Meaning
to catch up with and pass someone or something; to surpass
Example
The runner managed to overtake his competitor in the final lap.
A2 adjective /ˈaʊtdɔːr/

outdoor

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

occur

Meaning
to happen or take place; to be found or present
Example
The accident occurred at the intersection during rush hour.
C1 adjective /ˌoʊvərˈrɔt/

overwrought

Meaning
extremely agitated or nervous; excessively elaborate
Example
She was too overwrought to speak clearly after the accident.
A2 adjective /ɒd/

Odd

Meaning
strange; unusual; different from what is normal or expected
Example
That's an odd way to solve the problem.
C2 noun /ˈɑːbəlɪsk/

obelisk

Meaning
A tall, four-sided, narrow tapering monument with a pyramid-shaped top.
Example
The ancient Egyptians built obelisks to honor their gods.
C1 verb /ˌaʊtˈduː/

outdo

Meaning
To do better than someone else; to surpass.
Example
She always tries to outdo her classmates in exams.
B1 noun əbˈdʒek.tɪv

objective

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

optics

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

outstretch

Meaning
To extend or stretch something outward.
Example
He outstretched his hand to help her up.
B2 adverb /ˈaʊtraɪt/

Outright

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

overt

Meaning
Done or shown openly; not hidden or secret.
Example
There was an overt display of hostility between the rivals.
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 /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.
B2 adjective /ˈɔː.də.li/

Orderly

Meaning
well-organized; systematic; peaceful
Example
Please keep your desk orderly and clean.
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.
B2 noun /ˈoʊ.dər/

Odour

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