The juxtaposition of two words that seem to be mutually contradictory: "Sweet sorrow", "thunderous silence", etc., usually used for rhetorical effect.
The idea can be stretched for humorous effect: Military Intelligence, United Kingdom, MicrosoftWorks, Popular Science, jumbo shrimp, honest politician, American History, Safe Sex, British Humour...
Or it can be used to illustrate when an idea is at odds with itself: Live Test
The equivalent of the hydroxyl group village idiot -- S
See also www.everything2.com 
- Which points out that the term in its original root sense is an "oxymoron".
Used by those opposed to what the word expresses and for the effect of criticism, to limit or remove the expression from use by a mockery. As in the expressions used above: Microsoft Works, Popular Science, Military Intelligence. The labelling is the result of a deliberate failure to see that the combination is AdjectiveNoun and has definite meaning contrary to the applied critical labelling. Usually demonstrating bias and predisposed opposition. Microsoft Works is a product, Popular Science is a magazine, Military Intelligence is a group of people charged with refining raw information into actionable information. To label them as oxymorons may be correct in the critics eyes, but denies the fact that the word does mean something definite. -- Anonymous On Purpose
...or else.
I do not believe that "honest politician" is really a proper oxymoron... rather more of an impossibility. The rest work quite well though.
See: Extreme Oxymoron
See original on c2.com