Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class V. Words Releasing to the Voluntary Powers
Division (I) Individual Volition
Section IV. Antagonism
2. Active Antagonism

710. Opponent.

   NOUN:OPPONENT, antagonist, adversary, oppugnant [rare]; adverse party, opposition; enemy [See Enemy]; assailant.
  OPPOSITIONIST, obstructive; brawler, wrangler, brangler [rare], disputant; filibuster [U. S.], filibusterer [U. S.], extremist, bitter-ender [U. S.], irreconcilable, obstructionist, “willful men.”
  MALCONTENT; demagogue, reactionist; anarchist, anarch; Jacobin, Fenian, Sinn Feiner; Red; Industrial Workers of the World, I. W. W.
  RIVAL, competitor, contestant, entrant; the field.
  1. Each brave foe was in his heart a friend.—Homer
  2. He that wrestles with us strengthens our nerves and sharpens our skill. Our antagonist is our helper.—Burke