octave
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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.
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.
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.
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.
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.
C1 noun /ˈoʊ.vər spɪl/

Over-spill

Meaning
the overflow of liquid from a container; excess that cannot be contained
Example
The overspill from the river flooded nearby farms.
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 noun /ˈɒbsɪkwi/

obsequy

Meaning
A funeral rite, usually used in plural form obsequies.
Example
The village gathered for the obsequy of their beloved teacher.
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 /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.
C1 verb /aʊtˈlæst/

outlast

Meaning
to continue longer than; to endure beyond someone or something else
Example
The old castle has outlasted many generations.
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 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 /əbˈsiːkwɪəs/

obsequious

Meaning
Excessively obedient or attentive in a servile way.
Example
The waiter gave an obsequious bow to every customer.
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.
A2 noun /ˈɒbdʒɪkt/

object

Meaning
a thing that can be seen and touched
Example
The table is an object in the room.
B2 adjective /ˈɒp.tɪ.məl/

optimal

Meaning
Best or most favorable; the most effective under particular conditions.
Example
The optimal temperature for plant growth is around 25°C.
C2 adjective /ɒˈstɛnsɪv/

ostensive

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

objurgation

Meaning
a harsh scolding or strong rebuke
Example
His misconduct earned him a severe objurgation from the manager.
C2 adjective /əˈrækjələr/

oracular

Meaning
Resembling an oracle; giving wise, mysterious, or prophetic statements
Example
Her oracular words left the audience deep in thought.
C1 noun /ˈɒktəɡən/

octagon

Meaning
A polygon with eight sides and eight angles.
Example
The stop sign is shaped like an octagon.
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.
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 adverb /ˈaʊtraɪt/

Outright

Meaning
completely; directly; without reservation or concealment
Example
She rejected the proposal outright.
C1 noun əˈblɪv.i.ən

oblivion

Meaning
The state of being unaware or unconscious of what is happening; complete forgetfulness.
Example
His name faded into oblivion over time.
C2 noun /ˈɒbsɪkwiːz/

obsequies

Meaning
Funeral rites or ceremonies.
Example
The obsequies for the late leader drew thousands of mourners.
B2 noun /ˈɔːrnəmənt/

ornament

Meaning
A decorative object that adds beauty to something.
Example
The Christmas tree was decorated with colorful ornaments.
C2 adjective /ˈɒpjʊlənt/

opulent

Meaning
Ostentatiously rich, luxurious, or lavish.
Example
They hosted an opulent wedding with gold decorations.
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.
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.
B1 verb /ˈɔːrɡənaɪz/

organize

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

officious

Meaning
Excessively eager to offer unwanted help or advice; meddlesome.
Example
The officious guard kept interrupting the visitors with unnecessary instructions.
B2 verb /ˌoʊvərˈiːt/

overeat

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

offend

Meaning
to cause displeasure or resentment; to hurt someone's feelings
Example
His rude comments offended many people at the meeting.
C2 adjective /ɑːlˈfæktəri/

olfactory

Meaning
Relating to the sense of smell.
Example
The perfume activated her olfactory nerves immediately.
A1 noun /ɒkˈtoʊbər/

october

Meaning
the tenth month of the year in the Gregorian calendar
Example
The weather becomes cooler in October.
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 noun /ˈaʊtˌkraɪ/

outcry

Meaning
A strong expression of public anger or disapproval.
Example
The decision sparked an outcry among the citizens.
C2 adjective /ɔːrˈθɑːɡənl/

orthogonal

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

open

Meaning
to move something so as to allow access; to begin
Example
Please open the door for me.
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.
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 noun /ˈɔr.fən/

Orphan

Meaning
a child whose parents are dead; someone without protective support
Example
The orphan was adopted by a loving family.
C1 noun /ˈɒn.slɔːt/

onslaught

Meaning
A fierce or destructive attack.
Example
The village faced an onslaught of enemy forces.
A2 noun /ɔɪl/

Oil

Meaning
a viscous liquid derived from petroleum or plants, used for cooking or fuel
Example
Heat the oil in a pan before adding the vegetables.
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.
C1 adverb /ˈaʊtwədli/

Outwardly

Meaning
on the surface; externally; in appearance
Example
Outwardly, he seemed calm, but inside he was nervous.
C2 adjective /ˈɒfɪʃ/

offish

Meaning
Unfriendly, distant, or reserved in manner.
Example
He seemed offish at the party and avoided most conversations.
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.
B1 adjective /əˈrɪdʒ.ɪ.nəl/

Original

Meaning
first; fundamental; primary; authentic
Example
This is the original document, not a copy.
C1 adjective /ˌɑːpərˈtuːn/

opportune

Meaning
Happening at a suitable or convenient time.
Example
She arrived at an opportune moment to help.
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 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 /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 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 /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 ˌɒp.tɪ.maɪˈzeɪ.ʃən

optimization

Meaning
Improving processes to increase performance and efficiency.
Example
SEO optimization helps websites rank higher on search engines.
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 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.
C1 adjective /ˈɒptɪməm/

optimum

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

opine

Meaning
to express an opinion or belief
Example
The professor opined that the policy would fail.
C1 adjective /ɒmˈnɪv.ər.əs/

Omnivorous

Meaning
eating both plants and animals; feeding on various kinds of food
Example
Bears are omnivorous animals that eat both plants and meat.
A2 modal verb /ɔːt/

ought

Meaning
Used to indicate duty, correctness, or moral obligation.
Example
You ought to respect your elders.
B2 verb /ˌoʊvərˈkʌm/

overcome

Meaning
To succeed in dealing with a problem or difficulty.
Example
You can overcome procrastination with planning.
C2 verb /ˌoʊvərˈstraɪd/

overstride

Meaning
To stride over something; to step across with long steps.
Example
He managed to overstride the narrow stream with ease.
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 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 adjective /ˈəʊ.dər.əs/

Odorous

Meaning
having a strong smell; fragrant; aromatic
Example
The garden was filled with odorous flowers.
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.
C1 adjective /əbˈnɒkʃəs/

Obnoxious

Meaning
extremely unpleasant; offensive; annoying
Example
His obnoxious behavior at the party annoyed everyone.
C1 adjective /ˌoʊvərˈbloʊn/

overblown

Meaning
excessively exaggerated or inflated
Example
The controversy was overblown by the media.
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.
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.
B1 noun /ˌɑk.jəˈpeɪ.ʃən/

Occupation

Meaning
a job or profession; the action of taking control of a place
Example
Teaching is her chosen occupation.
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.
B2 noun /ˈaʊtlet/

outlet

Meaning
a means by which something is released, expressed, or distributed
Example
Art is a great outlet for emotions.
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 /ˌɔːrθəˈpiːdɪk/

orthopedic

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

opaque

Meaning
Not able to be seen through; not transparent; difficult to understand.
Example
The glass was too opaque to see through.
B1 verb /oʊ/

owe

Meaning
to be under obligation to pay or repay; to be indebted
Example
I owe my success to my parents' support.
C1 noun /oʊˈveɪʃən/

ovation

Meaning
an enthusiastic show of approval, especially by clapping hands
Example
The actor received a standing ovation after his performance.
B2 noun/verb /ˈaʊtreɪdʒ/

outrage

Meaning
A strong feeling of anger and shock; to arouse anger or shock.
Example
The unfair decision caused public outrage.
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.
C1 verb ˈɔː.kɪ.streɪ.tɪd

orchestrated

Meaning
Planned or coordinated carefully and deliberately.
Example
The attacks were orchestrated with precision.
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 /ˈoʊpənɪŋ/

opening

Meaning
an act or instance of beginning something
Example
The opening of the new store was a huge success.
C1 verb /aʊst/

oust

Meaning
To remove someone from a position or place of power or authority.
Example
The rebels ousted the dictator from power.
C2 adjective /əˈprəʊ.bri.əs/

opprobrious

Meaning
Expressing strong criticism or scorn.
Example
He made opprobrious remarks about his opponent.
A1 preposition /ɒn/

on

Meaning
physically in contact with and supported by a surface
Example
The book is on the table.
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 /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.
C2 noun /ˌɒstɛnˈteɪʃən/

ostentation

Meaning
excessive display of wealth or knowledge intended to attract admiration or envy
Example
The billionaire’s party was full of ostentation and luxury.
C2 noun /ɒmˈnɪʃəns/

omniscience

Meaning
the state of knowing everything
Example
The novel's narrator has an air of omniscience, revealing every character's thoughts.
B2 verb /əˈrɪdʒɪneɪt/

originate

Meaning
to come into existence; to begin; to create or produce
Example
The tradition originated in ancient times.
A2 adjective /ˈɔː.dɪ.nər.i/

Ordinary

Meaning
normal; usual; not special
Example
It was just an ordinary day at the office.
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.
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.
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.
C2 adjective /ˈɒkjʊlər/

ocular

Meaning
Relating to the eye or vision.
Example
The patient was referred to a specialist for ocular treatment.
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.
C1 noun /ˌɑːsɪˈleɪʃən/

oscillation

Meaning
The movement back and forth at a regular speed.
Example
The oscillation of the pendulum controls the clock.
C2 verb /ˈɑːbdʒərˌɡeɪt/

objurgate

Meaning
to scold or rebuke severely
Example
The teacher objurgated the student for being dishonest.
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.
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 adjective /ˈɔɪl.i/

Oily

Meaning
covered with oil; containing oil; greasy
Example
Her oily skin needs special care.