old - ad. not young or new; having lived or existed for many years

on - prep. above and held up by; touching¹ the upper surface of ("The book is on the table."); supported by ("He is on his feet."); about ("The report on the meeting is ready."); at the time of ("He left on Wednesday.")

once - ad. one time only

only - ad. being the single one or ones; no more than ("We have only two dollars.")

open - v. to start ("They opened talks."); ad. not closed; not secret

operate - v. to do work or a job; to cut into the body for medical reasons

opinion - n. a belief based on one's own ideas and thinking

oppose - v. to be against; to fight against

opposite - ad. different as possible; completely different from; exactly the other way ("North is the opposite direction from south.")

oppress² - v. to make others suffer; to control by the use of unjust³ and cruel force or power

or - conj. giving another of two choices; giving the last of several choices

orbit⁴ - v. to travel in space around a planet⁵ or other object; n. the path or way an object travels in space around another object or planet

order - v. to give a command; to tell someone what to do; n. a command; the correct or normal way things are organized; a peaceful situation in which people obey laws

organize - v. to put in order; to put together into a system

other - ad. different; of another kind; the remaining one or ones of two or more ("That man is short; the other is tall.")

our - ad. of or belonging to us

oust⁶ - v. to force to leave; to remove by force

out - ad. away from the inside; opposite of in

over - conj. above; covering; across, in or on every part of ("all over the world")

overthrow⁷ - v. to remove from power; to defeat or end by force

owe - v. to pay or have to repay⁸ (usually money) in return for something received

own - v. to have or possess for oneself