The Collaborative International Dictionary of English v.0.48:

Oppose \Op*pose"\, v. t. [imp. & p. p. Opposed; p. pr. & vb.
   n. Opposing.] [F. opposer. See Ob-, Pose, and cf.2d
   Appose, Puzzle, n. Cf.L. opponere, oppositum.]
   1. To place in front of, or over against; to set opposite; to
      exhibit.
      [1913 Webster]

            Her grace sat down . . .
            In a rich chair of state; opposing freely
            The beauty of her person to the people. --Shak.
      [1913 Webster]

   2. To put in opposition, with a view to counterbalance or
      countervail; to set against; to offer antagonistically.
      [1913 Webster]

            I may . . . oppose my single opinion to his.
                                                  --Locke.
      [1913 Webster]

   3. To resist or antagonize by physical means, or by
      arguments, etc.; to contend against; to confront; to
      resist; to withstand; as, to oppose the king in battle; to
      oppose a bill in Congress.
      [1913 Webster]

   4. To compete with; to strive against; as, to oppose a rival
      for a prize.
      [1913 Webster]

            I am . . . too weak
            To oppose your cunning.               --Shak.
      [1913 Webster]

   Syn: To combat; withstand; contradict; deny; gainsay; oppugn;
        contravene; check; obstruct.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

opposed \opposed\ ([o^]p*p[=o]zd"), opposing \opposing\
   ([o^]p*p[=o]"z[i^]ng),adj.
   1. characterized by active opposition; as, two bitterly
      opposing schools of thought.

   Syn: antagonistic, antipathetic, antipathetical, opponent.
        [WordNet 1.5]

   2. acting in opposition to; as, the opposing sector of the
      same muscle group.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

opposing
    adj 1: characterized by active hostility; "opponent (or
           opposing) armies" [syn: opponent, opposing]

Moby Thesaurus II by Grady Ward, 1.0:

213 Moby Thesaurus words for "opposing":
   abjuratory, abnegative, adversary, adversative, adverse, adversive,
   against, alien, alienated, allegory, analogy, antagonistic,
   anteposition, anti, antiestablishment, antipathetic, antipodal,
   antithesis, antithetic, antithetical, antonymous,
   at cross-purposes, at odds with, at variance with, balancing,
   breakaway, bucking, challenge, clashing, comparative anatomy,
   comparative degree, comparative grammar, comparative judgment,
   comparative linguistics, comparative literature,
   comparative method, compare, comparing, comparison, compensating,
   competitive, con, conflicting, confrontation, confronting,
   confrontment, contention, contradicting, contradiction,
   contradictory, contradistinct, contraposition, contrapositive,
   contrariety, contrarious, contrary, contrast, contrasted,
   contrasting, contrastiveness, contravention, contraversion,
   converse, correlation, counter, counter-culture, counteractant,
   counteracting, counteraction, counteractive, counterbalancing,
   counterpoised, countervailing, counterworking, cranky, cross,
   crosscurrent, crossing, crotchety, dead against, denial, denying,
   detrimental, differing, difficult, disaccordant, disaffirming,
   disagreeing, disallowing, disappointed, disapprobatory,
   disapproving, disavowing, disclaiming, discontented, discordant,
   discrepant, disenchanted, disgruntled, disillusioned, disowning,
   displeased, dissatisfied, dissentient, dissenting, dissident,
   distinction, distinctiveness, enemy, eyeball to eyeball,
   eyeball-to-eyeball, facing, fractious, fronting, hard, harmful,
   head wind, hostile, hostility, impugnation, impugnment,
   in opposition, incompatible, inconsistent, indignant, inimical,
   inverse, irreconcilable, likening, low, matching, metaphor,
   miserable, negation, negative, negatory, nonconforming,
   nonconformist, noncooperative, not easy, obstinate, obverse,
   opponent, opposed, opposite, opposition, oppositional, oppositive,
   opposure, oppugnant, oppugnation, overthwart, parallelism,
   perverse, polar, polar opposition, polaric, polarity, polarization,
   polarized, poor, posing against, proportion, reactionary,
   rebutment, rebuttal, recalcitrant, recanting, recusant, refractory,
   refusal, rejection, relation, renitent, renunciative, renunciatory,
   repudiative, repugnant, resistance, resistant, reverse, revocative,
   revocatory, revolutionary, rigorous, rival, sectarian, sectary,
   simile, similitude, sinister, squared off, standing against,
   stressful, traversal, trope of comparison, troublesome, troublous,
   trying, turned-off, unappreciative, unapproving, uncomplimentary,
   uncooperative, undercurrent, underground, unfavorable, unfriendly,
   unhappy, unpropitious, untoward, weighing, wretched