The Collaborative International Dictionary of English v.0.48:

Opponent \Op*po"nent\, a. [L. opponens, -entis, p. pr. of
   opponere to set or place against, to oppose; ob (see Ob-) +
   ponere to place. See Position.]
   Situated in front; opposite; hence, opposing; adverse;
   antagonistic. --Pope.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Opponent \Op*po"nent\, n.
   1. One who opposes; an adversary; an antagonist; a foe.
      --Macaulay.
      [1913 Webster]

   2. One who opposes in a disputation, argument, or other
      verbal controversy; specifically, one who attacks some
      thesis or proposition, in distinction from the respondent,
      or defendant, who maintains it.
      [1913 Webster]

            How becomingly does Philopolis exercise his office,
            and seasonably commit the opponent with the
            respondent, like a long-practiced moderator! --Dr.
                                                  H. More.
      [1913 Webster]

   Syn: Antagonist; opposer; foe. See Adversary.
        [1913 Webster]

WordNet (r) 3.0 (2006):

opponent
    adj 1: characterized by active hostility; "opponent (or
           opposing) armies" [syn: opponent, opposing]
    n 1: a contestant that you are matched against [syn:
         opposition, opponent, opposite]
    2: someone who offers opposition [syn: adversary,
       antagonist, opponent, opposer, resister] [ant:
       agonist]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "opponent":
   adversary, adversative, adverse, alien, antagonist, antagonistic,
   anti, antipathetic, antithetic, assailant, clashing, combatant,
   competitive, competitor, con, conflicting, contender, contestant,
   contradictory, contrary, counter, counteragent, cross,
   disaccordant, disputant, dissentient, enemy, foe, fractious,
   hostile, inimical, match, negative, noncooperative, obstinate,
   opposed, opposer, opposing, opposite, oppositional, oppositive,
   oppugnant, overthwart, perverse, recalcitrant, refractory,
   repugnant, rival, the opposition, uncooperative, unfavorable,
   unfriendly, unpropitious