An oxymoron is a literary or rhetorical device in which two contradictory terms are used together for emphasis or poetic effect or to arrive at a unique meaning. A few of the most commonly cited ones are deafening silence, living dead, open secret, and controlled chaos. The word came to English via Latin from Greek, where it came from an adjective meaning pointedly foolish.
English speakers are free to pluralize oxymoron in the Latin manner—oxymora—and some do, but the word has been in English many centuries and its English plural, oxymorons, is well established and far more common than the Latin one.
An oxymoron is a rhetorical device usually used intentionally, so, strictly speaking, it is not simply a synonym of contradiction, and it shouldn’t refer to a contradictory situation. It also shouldn’t refer to phrases that become unintentionally contradictory when considered a certain way (civil war and government worker, for instance). Yet these uses of the word are widespread and can’t be stopped, so even though we might question how the word is used in the following examples, these writers are just going along with popular usage:
I think he’s guilty of perjury and “obstructing Congress” (an oxymoron if ever there’s been one), regardless of the jury’s verdict this week. [Poughkeepsie Journal]
The following game-changers might just warm your view of the season and prove that “winter fun” is no oxymoron. [CNN International]
He was an extraordinary oxymoron of visionary and headkicker, with a sense of mission and an admirably clear way of expressing himself. [The Australian]
A democratic renewal led by people who can’t meet the eyes of their fellow citizens is an oxymoron.[Irish Times]
There is a a school of thought that “Knicks management” is, in fact, an oxymoron. [Guardian]