I came across a Wikipedia page a few months ago that described a literary device that had two opposite words side by side in a sentence.
Unlike an oxymoron ("horribly kind", "run slow"), this page described words that were almost identical, but one word was the "un" of the other word. For example:
- She was faithfully unfaithful to him
It is still a type of oxymoron, but I know the word I was thinking of to describe them that I read on the Wiki page was different.
Note: I'm not thinking of a tautology either. In the sentence above, "faithfully" adds more context to the unfaithful - she was faithful to her unfaithfulness.