Opponent, antagonist, adversary all denote one who expresses or manifests opposition.
Unlike enemy they do not necessarily imply personal animosity or hostility.
An opponent is one who is on the opposite side in a contest (as an argument, disputation, or election) or in a conflict (as of opinion).
Antagonist implies sharper opposition, especially in a struggle or combat for supremacy or control.
Adversary ranges in connotation from the idea of mere opposition to that of active hostility.