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, UnitedKingdom, 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: LiveTest
The equivalent of the hydroxyl group village idiot -- S
See also http://www.everything2.com/index.pl?node_id=16105 - 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: MicrosoftWorks, PopularScience, MilitaryIntelligence. 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. MicrosoftWorks is a product, PopularScience is a magazine, MilitaryIntelligence 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. -- AnonymousOnPurpose
<aside, to self> Buzzkill.
...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: ExtremeOxymoron