observatory
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C1 noun /əbˈzɜːrvətɔːri/

observatory

Meaning
A building equipped for observing astronomical or meteorological events.
Example
The observatory is open to the public on weekends.
A1 interjection /oʊ/

oh

Meaning
used to express surprise, realization, or understanding
Example
Oh, I didn't know you were here!
C1 adjective /ˌoʊvərˈbloʊn/

overblown

Meaning
excessively exaggerated or inflated
Example
The controversy was overblown by the media.
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.
B2 noun /ˌɑb.lɪˈɡeɪ.ʃən/

Obligation

Meaning
a duty or commitment; something that must be done legally or morally
Example
Parents have an obligation to care for their children.
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 adverb /əˈfɪʃəli/

officially

Meaning
in a formal or authorized way
Example
The building was officially opened by the mayor.
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 adverb /ˈɒp.tɪ.məl.i/

optimally

Meaning
In the most effective or efficient way possible.
Example
Tasks should be allocated optimally for maximum output.
C2 noun /əˈprəʊ.bri.əm/

opprobrium

Meaning
Harsh criticism or public disgrace.
Example
The scandal brought opprobrium on the politician.
B1 adjective /əˈbiː.di.ənt/

Obedient

Meaning
willing to comply with orders or instructions; submissive
Example
The obedient student always follows the teacher's instructions.
C1 noun /ˈɔːrθədɑːksi/

orthodoxy

Meaning
traditional or generally accepted beliefs and practices
Example
The religious leader defended the orthodoxy of the faith.
C1 adjective /ˌɑːpərˈtuːn/

opportune

Meaning
Happening at a suitable or convenient time.
Example
She arrived at an opportune moment to help.
B2 adverb /ˌoʊvərˈnaɪt/

Overnight

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

ordinance

Meaning
An authoritative rule, law, or decree issued by a city, town, or authority.
Example
The city council passed an ordinance banning smoking in public parks.
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.
A2 adjective/adverb /ˈɒnˌlaɪn/

online

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

outstretch

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

offering

Meaning
something that is given, especially as a gift or contribution
Example
The company’s latest offering is an eco-friendly car.
C2 verb /ˈɔːreɪt/

orate

Meaning
To speak in a formal, often pompous manner
Example
He began to orate at the ceremony.
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.
C1 adjective /əˈbliːk/

Oblique

Meaning
slanting; diagonal; indirect or evasive
Example
The artist drew an oblique line across the canvas.
A1 conjunction /ɔːr/

or

Meaning
used to link alternatives
Example
You can have tea or coffee.
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.
B1 verb /ˈɒpəreɪt/

operate

Meaning
to function; to control or manage; to perform surgery
Example
The surgeon will operate on the patient tomorrow.
A2 adjective /ˈɒb.vi.əs/

Obvious

Meaning
easily perceived or understood; clear and apparent to the mind
Example
It was obvious that she was upset about something.
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.
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.
B2 adverb /ˈɒdli/

oddly

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

Observation

Meaning
the action of watching something carefully; a comment or remark
Example
The scientist made careful observations of the experiment.
C2 noun /ˈɒn.rʌʃ/

onrush

Meaning
A sudden powerful forward or downward rush or flow.
Example
The onrush of water swept away the bridge.
C2 adjective /ˈɑːbdjʊrət/

obdurate

Meaning
Stubbornly refusing to change one's opinion or course of action.
Example
He remained obdurate despite everyone's pleas.
B1 verb /əbˈzɜːrv/

observe

Meaning
to watch carefully; to notice or perceive something
Example
Scientists observe the behavior of animals in the wild.
B1 noun əbˈdʒek.tɪv

objective

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

outset

Meaning
the beginning or start of something
Example
From the outset, the project seemed ambitious but achievable.
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 noun /ˌɒpəˈrɛtə/

operetta

Meaning
a short opera with light and humorous themes
Example
The theater staged a famous operetta by Gilbert and Sullivan.
C2 noun /oʊˈbeɪsəns/

obeisance

Meaning
A gesture of respect such as a bow or curtsy.
Example
They bowed in obeisance to the king.
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.
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.
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 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 /ˌɑːp.təˈmɪs.tɪk/

optimistic

Meaning
hopeful and confident about the future
Example
She is optimistic about the success of her new project.
C2 verb /ˈɑːbdʒərˌɡeɪt/

objurgate

Meaning
to scold or rebuke severely
Example
The teacher objurgated the student for being dishonest.
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 /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 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.
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.
C2 adjective /ˈəʊ.dər.əs/

Odorous

Meaning
having a strong smell; fragrant; aromatic
Example
The garden was filled with odorous flowers.
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.
B2 adjective /ˈɒp.ʃən.əl/

Optional

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

overseer

Meaning
a person who supervises work or workers; a manager
Example
The overseer made sure the workers followed safety rules.
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.
C2 adjective /ˈoʊdiəs/

odious

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

Observant

Meaning
quick to notice things; watchful; attentive
Example
The observant detective noticed every small detail at the crime scene.
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 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.
C2 noun /oʊf/

Oaf

Meaning
a clumsy, stupid, or insensitive person
Example
Don't be such an oaf - handle the dishes carefully!
C2 noun /ɒmˈnɪpətəns/

omnipotence

Meaning
The quality of having unlimited power or authority.
Example
Many religions describe God’s omnipotence as absolute.
C2 noun /ˌɔːrnɪˈθɑːlədʒi/

ornithology

Meaning
the scientific study of birds
Example
She pursued ornithology to understand the migration patterns of birds.
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.
A2 modal verb /ɔːt/

ought

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

overpay

Meaning
To pay more than is necessary or reasonable.
Example
She realized she had overpaid for the concert tickets.
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 noun ˌɔː.ɡən.aɪˈzeɪ.ʃənz

organizations

Meaning
Organized groups of people with a particular purpose
Example
Organizations play a key role in policymaking.
B2 adjective /aʊtˈstændɪŋ/

outstanding

Meaning
extremely good or impressive; not yet resolved or paid
Example
She received an award for her outstanding performance.
C2 noun /oʊliˈændər/

Oleander

Meaning
a poisonous evergreen shrub with clusters of white, pink, or red flowers
Example
The beautiful oleander flowers bloomed in the garden despite their toxic nature.
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.
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.
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 /aʊtˈmoʊdɪd/

outmoded

Meaning
No longer in fashion or useful; outdated.
Example
That old computer is completely outmoded.
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.
C1 adjective /ˌoʊvərˈrɔt/

overwrought

Meaning
extremely agitated or nervous; excessively elaborate
Example
She was too overwrought to speak clearly after the accident.
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.
A1 verb /ˈoʊpən/

open

Meaning
to move something so as to allow access; to begin
Example
Please open the door for me.
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.
C1 verb ˈɔː.kɪ.streɪ.tɪd

orchestrated

Meaning
Planned or coordinated carefully and deliberately.
Example
The attacks were orchestrated with precision.
A1 preposition /ɒn/

on

Meaning
physically in contact with and supported by a surface
Example
The book is on the table.
B2 adjective ˌɒp.ərˈeɪ.ʃən.əl

operational

Meaning
In working order; ready for use.
Example
The space project will be operational by 2025.
C1 noun/verb /ˈoʊvərˌhæŋ/

overhang

Meaning
To extend outward and hang over something; a projecting part.
Example
The roof overhang provides shade in the summer.
B2 noun /ˈaʊtˌbreɪk/

outbreak

Meaning
a sudden start or occurrence of something unpleasant, like disease or violence
Example
There was an outbreak of flu in the city.
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 verb /ˌoʊvərˈlʊk/

overlook

Meaning
to fail to notice or consider something; to have a view from above
Example
It's easy to overlook small details when you're in a hurry.
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 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.
A1 pronoun /ˈʌðərz/

others

Meaning
people or things distinct from the ones already mentioned or known
Example
I like to spend time with others.
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.
C2 adjective /əbˈsiːkwɪəs/

obsequious

Meaning
Excessively obedient or attentive in a servile way.
Example
The waiter gave an obsequious bow to every customer.
B1 adverb /əˈrɪdʒɪnəli/

originally

Meaning
in the beginning, at first
Example
Originally, she wanted to be a doctor.
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.
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 /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 /oʊˈkeɪ/

ok

Meaning
used to indicate agreement, approval, or understanding
Example
Okay, let's go to the park.
C1 noun /ˈɔr.ə.tər/

Orator

Meaning
a public speaker, especially one who is eloquent or skilled in speaking
Example
The famous orator delivered an inspiring speech.
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.
B2 adjective /aʊtˈreɪdʒəs/

outrageous

Meaning
Shockingly bad or excessive; very unusual or amusing.
Example
The comedian told an outrageous joke.
B1 adjective /ˈaʊtər/

outer

Meaning
situated on or toward the outside; external
Example
The outer layer of the jacket is waterproof.
B2 adjective /ˌfæʃ.ənd/

Outdated

Meaning
obsolete; no longer current
Example
This computer software is completely outdated.
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 /ˈɒpəreɪtər/

operator

Meaning
a person who operates equipment, a business, or controls something
Example
The machine operator ensured that everything ran smoothly.
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.
B1 verb অকার্স

occurs

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

overpass

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

ornamental

Meaning
Serving as decoration; decorative rather than functional.
Example
The garden was filled with ornamental plants.
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.
C2 adjective /əbˈstrɛpərəs/

obstreperous

Meaning
Noisy and difficult to control.
Example
The obstreperous students disrupted the class.