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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.
orbit
The path one body takes around another.
ordeal
A painful or difficult experience.
orderly
Neatly arranged or organized.
ore
A mineral or rock containing valuable metal.
organ
A part of the body with a specific function.
organism
A living thing, like a plant or animal.
organized crime
Illegal business run by large, well-organized criminal groups.
orientation
An introduction that helps someone become familiar with a place.
originally
at first; in the beginning
out of one's mind
acting very strangely or irrationally.
out of the blue
unexpectedly or without warning
out of the question
not possible or acceptable.
outweigh
To be greater in importance or significance.
over and over
Many times, again and again.
over my dead body
refusal to allow something to happen
overdue
not paid or returned on time
overgraze
To let animals eat so much plant cover that the land is harmed.
oxygen
A gas essential for breathing and combustion.
ozone
A gas that protects Earth from harmful sun rays.
Words selected by Inoh for the TOEFL iBT test. Not an official ETS list. How these words were chosen →