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]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "opposer":
   adversary, antagonist, anti, bitter-ender, brawler, con, defendant,
   diehard, disputant, dissentient, dissident, intransigent,
   irreconcilable, last-ditcher, litigant, match, naysayer,
   negativist, noncooperator, objector, obstructionist, obstructive,
   oppositionist, oppugnant, plaintiff, protester, quarreler,
   resister, scrapper, wrangler