Tautologies, such as tiny little
tautology
The antonym of an oxymoron is a tautology. For example: "almost exactly" is an oxymoron. "Tiny little" is a tautology.
Truism, tautology.
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.