The Collaborative International Dictionary of English v.0.48:

Oppugn \Op*pugn"\, v. t. [imp. & p. p. Oppugned; p. pr. & vb.
   n. Oppugning.] [OF. oppugner, L. oppugnare; ob (see Ob-)
   + pugnare to fight. See Impugn.]
   To fight against; to attack; to be in conflict with; to
   oppose; to resist.
   [1913 Webster]

         They said the manner of their impeachment they could
         not but conceive did oppugn the rights of Parliament.
                                                  --Clarendon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oppugn
    v 1: challenge the accuracy, probity, or propriety of; "We must
         question your judgment in this matter" [syn: question,
         oppugn, call into question]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "oppugn":
   antagonize, battle, be antipathetic, be at cross-purposes,
   be contrary to, be inimical, be opposed to, beat against, belie,
   call into question, challenge, clash, collide, conflict,
   conflict with, confute, contest, contradict, contrapose,
   contrast with, contravene, controvert, counter, counteract,
   counterattack, counterbalance, countercheck, counterpoise,
   counterpose, countervail, counterwork, cross, deny, fight, gainsay,
   go against, go counter to, interfere with, juxtapose in opposition,
   lock horns, meet head-on, militate against, negate, offset, oppose,
   play at cross-purposes, rebut, reject, resist, run against,
   run counter to, set off, swim upstream, traverse, tug, war,
   work against