An oxymoron (plural oxymorons or, more rarely, oxymora) is a figure of speech that combines two normally contradictory terms. Oxymoron is a loanword from Greek oxy ("sharp" or "pointed") and moros ("dull"). Thus the word oxymoron is itself an oxymoron.
Oxymorons are a proper subset of the expressions called contradictions in terms. What distinguishes oxymorons from other paradoxes and contradictions is that they are used intentionally, for rhetorical effect, and the contradiction is only apparent, as the combination of terms provides a novel expression of some concept, such as "cruel to be kind" or "team of mavericks".
The most common form of oxymoron involves an adjective-noun combination. For example, the following line from Tennyson's Idylls of the King contains two oxymorons:
Oxymorons can also be wooden irons in that they are in violation of the Principle of contradiction which asserts that nothing can be thought if it contains contradictory characteristics, predicates, attributes, or qualities.
Clearly, he could have used a different phrase, such as perhaps "hydrodynamics of fluids in the limiting case of viscosity approaching zero," but the deliberate contradiction of the phrase "dry water" both adds humor to his otherwise-dry analysis, and also emphasizes the fact that the substance he is discussing is theoretical and not real.Some examples of deliberate oxymorons include:
Oxymorons are most tellingly employed in injecting a sense of ironic, ostensibly unintended, humor. The effect is to confront the reader or the listener with a sense of ludicrousness so as to render the whole sentence and the idea absurd and funny. It should be remembered that this is a purely subjective line of thinking and presupposes that the reader or listener is already familiar with the intended humor. Examples of such thought-provoking oxymorons include:
Many collected lists of oxymorons are available, for example at oxymorons.info
and atlantamortgagegroup.com 
Very often the labeling of an expression as a perceived oxymoron is made on the basis of substituting an alternative, non-intended meaning for the meaning normally intended in the context of the expression in question. For instance, in the expression Civil war, the term civil is normally intended to mean "between citizens of the same state". In this sense, the expression is neither paradox nor self-contradictory. However, if civil is construed as 'non-military' or 'reasonable and polite', the expression is a contradiction in terms (as per satirist Richard Armour in It All Started with Columbus, who said the American Civil War was fought politely). Such designations of alleged oxymorons are often made with a humorous purpose. Alternatively, an oxymoron may occur when a word or phrase changes meaning. Few people today pay attention to the inherent contradiction in eating with "plastic silverware" or drinking from "a plastic glass," because the word "silverware" has come to mean eating utensils of any composition, and "glass" is commonly used to refer to any cup from which one can drink.
Calling such an expression an oxymoron is sometimes done in order to disparage its use, by drawing attention to a perceived inherent contradiction and thus claiming it to be nonsensical. Often this kind of argument is used in domains of political or ideological dispute, or in order to criticize a perceived nonsensical use of technical terms by lay people who fail to understand their true meanings. Examples of expressions that are used without a sense of paradox by some but have been claimed to be "oxymorons" in this sense by critics include:
A more subtle rhetorical maneuver in designating an expression XY as an "oxymoron", often used for either humorous or polemical purposes, is to pick out a perceived or alleged property of objects of type Y, re-construe that property as if it were a defining criterion of Y, and then demonstrate that it is contradicted by X. For instance, to claim "honest politician" is an oxymoron implies politicians are inherently dishonest.
Both the above strategies can be seen combined in an example like military intelligence, one of the many humorous oxymorons popularized by George Carlin; it carries and implies a political judgment, that the military by its nature cannot be intelligent. The term "intelligence" is re-construed as meaning not "information gathering" but "intellectual power." The Thrash metal band Megadeth commented on the phrase in their highly acclaimed song, Hangar 18: "Military Intelligence, two words combined that can't make sense".
For instance, some oxymorons used in this manner to disparage or arouse humor include:
Some phrases when put together, are considered oxymorons due to public perception and stereotypes:
Romeo. Why then, O brawling love! O loving hate!
O anything of nothing first create,
A heavy lightness, serious vanity,
Misshapen chaos of well-seeming forms,
Feather of lead...
Juliet also uses oxymorons after having found out about her cousin's death at the hands of Romeo she says:
O serpent heart, hid with a flow'ring face!
Beautiful tyrant, fiend angelical'
Dove-feathered raven, wolfish ravening lamb'
A damned saint, an honourable villain