The two words have totally different meanings. We all know what “opposite” means. When you say that your brother is the opposite of you, what you mean is that he is very different from you. “Apposite”, on the other hand, means “appropriate” or “relevant”. When you say that something is apposite, what you are implying is that it is suitable to what is happening or being discussed. The “a” in the first syllable is like the “a” in “apple”. The word is considered to be formal. Both “opposite” and “apposite” have the stress on the first syllable.
*The argument that the student came up with was apposite to the case. *It was Amrit who came up with an apposite title for the book.