Oxymoron does put two seemingly contradictory terms together but for a very different effect. Its purpose is to show that in special circumstances such contradictions actually make sense. The term comes from the Greek and means "pointedly foolish" - so not foolish at all when looked at in the special way intended by the writer. Consider the following examples:
1. The twenty fools at the table showed craven fear through their absurd responses to the imminent attack; Fred, on the other hand, spoke with eloquent silence.
2. O heavy lightness! serious vanity!
Misshapen chaos of well-seeming forms!
Feather of lead, bright smoke, cold fire, sick health!"
(This, by Romeo, is nonsensical contradiction until you understand that Romeo is describing his experience of first love and the joyful pain it brings).
Typically, the oxymoron is composed of an adjective and noun and its effect is that of a compressed paradox. If I say "the pain I feel is joy" or "Fred's silence surpassed in eloquence the empty rhetoric of his fellow councillors," I am using statement (a full clause) to create the same effect. This is paradox.
Remember: oxymoron is not a contradiction in terms; rather, it is a miracle in language, making the impossible possible - it is a contradiction which makes sense!