The Collaborative International Dictionary of English v.0.48:

Obviate \Ob"vi*ate\, v. t. [imp. & p. p. Obviated; p. pr. &
   vb. n. Obviating.] [L. obviare; ob (see Ob-) + viare to
   go, fr. via way. See Voyage.]
   [1913 Webster]
   1. To meet in the way. [Obs.]
      [1913 Webster]

            Not to stir a step to obviate any of a different
            religion.                             --Fuller.
      [1913 Webster]

   2. To anticipate; to prevent by interception; to remove from
      the way or path; to make unnecessary; as, to obviate the
      necessity of going.
      [1913 Webster]

            To lay down everything in its full light, so as to
            obviate all exceptions.               --Woodward.
      [1913 Webster]

WordNet (r) 3.0 (2006):

obviate
    v 1: do away with [syn: obviate, rid of, eliminate] [ant:
         ask, call for, demand, involve, necessitate,
         need, postulate, require, take]
    2: prevent the occurrence of; prevent from happening; "Let's
       avoid a confrontation"; "head off a confrontation"; "avert a
       strike" [syn: debar, forefend, forfend, obviate,
       deflect, avert, head off, stave off, fend off,
       avoid, ward off]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "obviate":
   anticipate, avert, bar, debar, deflect, deter, discourage,
   dishearten, estop, exclude, fend, fend off, forbid, foreclose,
   forestall, help, interfere, interpose, intervene, keep from,
   keep off, preclude, prevent, prohibit, repel, rule out, save,
   stave off, turn aside, ward, ward off