If one thing is antithetical to another, it is exactly opposite — not a bit different, but against it in spirit. A life of showing off your money is antithetical to a promise to live simply. The usual pattern is antithetical to. It is a formal, essay word. People reach for opposite or “clashes with” in ordinary talk. Though actually, speakers sometimes use it a little loosely, for “I don’t like this” or “this doesn’t fit,” when a true opposite would be a clean mirror. Teachers of writing like the strict sense. It describes a relationship between two things; it is not a mood word the way antipathetic is.