What's the opposite of oxymoron? That is, two words put together that seem identical?
oxymoron, noun : a combination for epigrammatic effect of contradictory or incongruous words
pleonasm, noun : the coincident use of a word and its substitute for the same grammatical function
tautology, noun : needless or meaningless repetition in close succession of an idea, statement, or word
I'm not quite sure how the use of one word can be coincident with another per M-W's definition, but looking at pleonasms.com, one sees these nuggets:
I'd say pleonasm comes the closest to being the antonym of oxymoron, in spirit if not in truth.
Try tautology. @alain-pannetier also made a good suggestion in the comments with pleonasm. The Wikipedia page explains the difference.
These are nice, but only somewhat related to what I assume the original question is getting at, that is: if a good example of an oxymoron is an expression that involves both a contradiction and an ironic contrast between terms in the expression; the opposite or the reverse of an oxymoron might be an expression which involves an ironic reinforcement between terms in the expression. Neither a pleonasm or tautology assert any sense of irony or humor, as is inherent in an oxymoron.
If "military intelligence" is a good example of an oxymoron, "contempt of Congress" might be an example of the opposite. An oxymoron might give a sense of "these things that don't go together;" the reverse might be the sense of "duh, clearly these are related."
There is no opposite, the word internalizes opposites, its own contradictions. It'd be like making a black & white negative image of a chessboard.
An oxymoron ... is a paradox compressed into a single self-contradicting phrase, and therefore the show-off among figures of speech. / Helen Vendler, The Music of What Happens 1988
... the discord of the oxymoron. Basil L Gildersleeve, Pindar: The Olympian and Pythian Odes