opposer

from WordNet (r) 3.0 (2006)

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

from 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]

from 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