Opposite refers to things of the same kind but that are completely "different" in a particular way.
A diffference refers to an element or factor that separates or distinguishes contrasting situations.
We can say that the amount of differences make sometimes things opposites. A maximum amount of differences of things in a specific area create an opposite, it is important to say that there are not so many differences that the words aren'r related at all. For example full and empty, this words describe the amount of things in a certain place, they refer to both ends, something at its maximum capacity or without anything, not giving place to something between those.