The Collaborative International Dictionary of English v.0.48:

opposer \op*pos"er\ ([o^]p*p[=o]z"[~e]r), n.
   One who opposes; an opponent; an antagonist; an adversary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opposer
    n 1: someone who offers opposition [syn: adversary,
         antagonist, opponent, opposer, resister] [ant:
         agonist]