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42,394 words to explore
1,392 words
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obsolete
No longer used or out of date.
obvious
Clear and easy to understand or see.
oceanographer
A scientist who studies the ocean and the life within it.
off the cuff
Said or done without any preparation beforehand.
off the hook
Released from a difficult situation.
offspring
The young born to a person or animal.
omnivore
An animal that eats both plants and meat.
on and off
occurring intermittently, not continuously
on cloud nine
to feel extremely happy or joyful
on one's way out
About to leave, or soon to disappear or become unpopular.
on pins and needles
feeling anxious or nervous
on second thought
After reconsideration or reflection.
on short notice
With very little warning or time to prepare.
on the blink
Not working properly, often breaking down from time to time.
on the cutting edge
At the newest and most advanced stage of development.
on the dot
Exactly at the scheduled time.
on the house
Given for free by a restaurant, bar, or business.
on the tip of one's tongue
Almost remembered, but not quite able to say it.
once in a blue moon
Very rarely or infrequently.
once in a while
occasionally, not often.
open up to
To start sharing your private feelings or thoughts with someone.
operant conditioning
Learning in which behavior changes because of rewards or punishments that follow it.
optional
not required; left to choice.
oral literature
Stories, poems, and songs passed down by speaking rather than writing.
Words selected by Inoh for the TOEFL iBT test. Not an official ETS list. How these words were chosen →