overstimulated
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C1 adjective /ˌəʊvərˈstɪmjʊˌleɪtɪd/

overstimulated

Meaning
Excessively excited or aroused, overwhelmed by too much stimulus.
Example
Overstimulated minds can lead to burnout and decreased productivity.
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 adjective /əˈblɪɡ.ə.tɔː.ri/

obligatory

Meaning
Required by a legal, moral, or other rule; compulsory.
Example
Wearing a seatbelt is obligatory in most countries.
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 noun /ˈɔːrkɪd/

Orchid

Meaning
a diverse family of flowering plants known for their exotic and often fragrant flowers
Example
The white orchid on the windowsill bloomed for months.
C1 adjective /ˈɒptɪk/

optic

Meaning
Relating to the eye or vision.
Example
The optic nerve carries signals from the eye to the brain.
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 adjective, verb /əbˈskjʊr/

obscure

Meaning
Not clearly expressed or understood; to make something difficult to see or understand.
Example
The meaning of the poem was obscure to most readers.
B1 adjective, adverb /ˈoʊvərɔːl/

overall

Meaning
in general or as a whole
Example
Overall, the project has been successful.
B1 noun əbˈdʒek.tɪv

objective

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

overlap

Meaning
To extend over and cover partly; an area where things coincide.
Example
The two meetings overlap by an hour.
B2 adjective ˌɒp.ərˈeɪ.ʃən.əl

operational

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

ordinal

Meaning
Relating to the order or position of something in a sequence (first, second, third, etc.).
Example
The ordinal number 'third' indicates position in a sequence.
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.
B2 noun /ˈɒkjʊpənt/

occupant

Meaning
A person who resides in or is present in a house, vehicle, seat, or place.
Example
The car had three occupants at the time of the accident.
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.
C2 noun /oʊˈbeɪsəns/

obeisance

Meaning
A gesture of respect such as a bow or curtsy.
Example
They bowed in obeisance to the king.
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.
C2 noun /ˌɒpəˈrɛtə/

operetta

Meaning
a short opera with light and humorous themes
Example
The theater staged a famous operetta by Gilbert and Sullivan.
B2 adjective /əˈfen.sɪv/

Offensive

Meaning
causing displeasure or resentment; insulting; aggressive
Example
His offensive remarks hurt everyone's feelings.
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.
A2 noun /ˈɒm.lət/

Omelet

Meaning
a dish made from beaten eggs cooked in a frying pan and often folded around a filling
Example
She made a cheese omelet for breakfast.
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.
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 noun /ɒpˈtɒmətri/

optometry

Meaning
The practice or profession of examining the eyes and prescribing corrective lenses.
Example
She studied optometry at the university to become an eye specialist.
B2 noun /ˌɒpəˈzɪʃən/

opposition

Meaning
the action of opposing or resisting something
Example
The opposition to the new law was strong.
C2 noun ˈɒpjʊləns

opulence

Meaning
Great wealth or luxuriousness.
Example
The opulence of the mansion was evident in its ornate furnishings.
C2 noun /ˌoʊpəˈlɛsəns/

opalescence

Meaning
a milky or pearly appearance of a surface like that of an opal
Example
The pearl had a gentle opalescence under the light.
C1 verb /ˈɒfsɛt/

offset

Meaning
To counterbalance or compensate for something.
Example
The company's losses were offset by gains in other departments.
A1 adjective /oʊˈkeɪ/

ok

Meaning
used to indicate agreement, approval, or understanding
Example
Okay, let's go to the park.
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 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.
A1 preposition, adverb /ˈoʊvər/

over

Meaning
across, on top of, or beyond
Example
The book is lying over the table.
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.
A1 adjective /əʊˈkeɪ/

okay

Meaning
used to express agreement or acceptance
Example
It's okay to leave early today.
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.
A1 adjective /ˈʌðər/

other

Meaning
used to refer to something different from the one already mentioned or known
Example
I don't want this one, I will take the other one.
B1 noun ˈɒk.sɪ.dʒən

oxygen

Meaning
A colorless, odorless reactive gas, the chemical element of atomic number 8 and the life-supporting component of the air.
Example
Oxygen is vital for life.
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 /əˈblɪkwɪti/

obliquity

Meaning
Deviation from moral or usual conduct; indirectness or lack of straightforwardness.
Example
The politician was criticized for the obliquity of his answers.
C2 adjective /ˈɒkjʊlər/

ocular

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

optometrist

Meaning
A healthcare professional who examines eyes and prescribes corrective lenses.
Example
The optometrist prescribed new glasses for her.
B2 adverb /ˈaʊtraɪt/

Outright

Meaning
completely; directly; without reservation or concealment
Example
She rejected the proposal outright.
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.
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.
A1 conjunction /ɔːr/

or

Meaning
used to link alternatives
Example
You can have tea or coffee.
B2 adjective /aʊtˈreɪdʒəs/

outrageous

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

operation

Meaning
an act or process of functioning or operating
Example
The hospital performed a successful operation.
C1 noun /ˈɔɪntmənt/

ointment

Meaning
A smooth, oily substance applied to the skin for healing or soothing.
Example
The doctor prescribed an ointment for the burn on his arm.
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.
C1 adjective /ˈaʊt.kɑːst/

Outcast

Meaning
rejected by society; excluded person
Example
He felt like an outcast in his new school.
C1 verb ˈɔː.kɪ.streɪ.tɪd

orchestrated

Meaning
Planned or coordinated carefully and deliberately.
Example
The attacks were orchestrated with precision.
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 adjective ˈɒk.sɪ.də.tɪv

oxidative

Meaning
Related to or caused by oxidation, especially cellular damage from free radicals.
Example
Excess stress can lead to oxidative damage in cells.
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.
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.
C1 adjective /əˈblaɪdʒɪŋ/

obliging

Meaning
Willing to help or do favors; accommodating.
Example
The staff at the hotel were very obliging and friendly.
C2 verb /ˈoʊɡəl/

ogle

Meaning
To look at someone with obvious sexual interest.
Example
He kept ogling the models on the runway.
C2 noun /ˈɒbsɪkwiːz/

obsequies

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

outpost

Meaning
A small military camp or position at some distance from the main force, used especially as a guard.
Example
The soldiers were stationed at a remote outpost.
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.
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.
A1 noun/adjective/adverb /ˌaʊtˈsaɪd/

outside

Meaning
the external part or surface of something; beyond the boundary
Example
Children were playing outside in the yard.
B2 adjective /ˈɒp.ʃən.əl/

Optional

Meaning
not compulsory; left to choice
Example
Wearing a tie is optional for this event.
C1 verb /aʊtˈlæst/

outlast

Meaning
to continue longer than; to endure beyond someone or something else
Example
The old castle has outlasted many generations.
C1 noun /ˈɒktəɡən/

octagon

Meaning
A polygon with eight sides and eight angles.
Example
The stop sign is shaped like an octagon.
C1 adjective /əbˈdʒek.ʃən.ə.bəl/

Objectionable

Meaning
arousing distaste or opposition; unpleasant or offensive
Example
His objectionable behavior made everyone uncomfortable.
C2 noun /oʊˈbleɪʃən/

oblation

Meaning
a religious offering, especially of bread or wine to God
Example
The priest placed the oblation on the altar during the ceremony.
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 /oʊˈveɪʃən/

ovation

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

outskirts

Meaning
The outer parts of a town or city, away from the center.
Example
They moved to the outskirts of the city for a quieter life.
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.
A1 adjective/adverb /ˈoʊnli/

only

Meaning
without others; solely
Example
He is the only person who knows the secret.
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 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.
C2 noun /ˌɔːrnɪˈθɒlədʒɪst/

ornithologist

Meaning
a scientist who studies birds
Example
The ornithologist spent years studying migratory birds.
C1 adjective /ˌɒmnɪˈprezənt/

omnipresent

Meaning
(of God) present everywhere at the same time.
Example
Social media has become an omnipresent force in modern society.
A1 verb /ˈoʊpən/

open

Meaning
to move something so as to allow access; to begin
Example
Please open the door for me.
C2 adjective /ˈɒpjʊlənt/

opulent

Meaning
Ostentatiously rich, luxurious, or lavish.
Example
They hosted an opulent wedding with gold decorations.
C1 adjective /ˈɒptɪməm/

optimum

Meaning
Most conducive to a favorable outcome; best.
Example
To achieve optimum results, we need to improve efficiency.
B2 adjective /ˈɒnɡəʊɪŋ/

ongoing

Meaning
continuing; still in progress
Example
The ongoing project will finish next month.
B2 noun /ˈɔːrɡənaɪzər/

organizer

Meaning
a person who arranges or coordinates activities, events, or groups; a tool or item used to keep things in order
Example
She worked as the main organizer of the international conference.
B1 verb /ˈɒpəreɪtɪŋ/

operating

Meaning
to control or manage the functioning of something
Example
The company has been operating in this market for five years.
C2 noun /əˈprəʊ.bri.əm/

opprobrium

Meaning
Harsh criticism or public disgrace.
Example
The scandal brought opprobrium on the politician.
C2 noun /ˈɒrɪz(ə)n/

orison

Meaning
a prayer, especially a devout or formal one
Example
He whispered a quiet orison before going to sleep.
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 adverb /ˌoʊvərˈnaɪt/

Overnight

Meaning
during the night; suddenly; very quickly
Example
The city was transformed overnight.
B2 adjective /ˈaʊtˌɡoʊɪŋ/

outgoing

Meaning
Friendly and sociable; inclined to mix with others.
Example
He is an outgoing person who loves meeting new people.
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 adjective əʊˈpeɪk

opaque

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

olfactory

Meaning
Relating to the sense of smell.
Example
The perfume activated her olfactory nerves immediately.
C1 verb /ˈoʊvərˌhɔːl/

overhaul

Meaning
to examine thoroughly and repair or improve something that is worn or ineffective
Example
The company decided to overhaul its outdated computer system.
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 /ˌaʊt ən ˈaʊt/

out-and-out

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

oaken

Meaning
made of or resembling oak wood
Example
The villagers gathered around the oaken table in the hall.
A1 adjective /oʊld/

old

Meaning
having lived for a long time; not new or young
Example
He still drives his old car.
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 adjective /ˌɑːp.təˈmɪs.tɪk/

optimistic

Meaning
hopeful and confident about the future
Example
She is optimistic about the success of her new project.
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 /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 /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.
C1 verb /ˌoʊvərˈθroʊ/

overthrow

Meaning
To remove a leader or government from power, often by force.
Example
The rebels planned to overthrow the corrupt regime.
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.
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 verb /əˈfɪʃieɪt/

officiate

Meaning
To perform official duties, especially in a ceremony or as a referee.
Example
The priest will officiate the wedding ceremony.
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 adjective /ˌoʊvərˈbɛrɪŋ/

overbearing

Meaning
unpleasantly or arrogantly domineering
Example
His overbearing attitude made everyone uncomfortable.
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.
B1 verb /ˈɔːrɡənaɪz/

organize

Meaning
to arrange systematically; to plan and coordinate
Example
She organized a surprise party for her friend.
C2 adjective /ˈɔːrəˌtʌnd/

orotund

Meaning
full, rich, and clear in sound; pompous in style
Example
The actor's orotund voice filled the theater.
C2 noun /ɑːlˈfækʃən/

olfaction

Meaning
The sense of smell; the process of detecting and perceiving odors.
Example
Dogs have a highly developed sense of olfaction.