The Collaborative International Dictionary of English v.0.48:

Omen \O"men\, n. [L. omen, the original form being osmen,
   according to Varro.]
   An occurrence supposed to portend, or show the character of,
   some future event; any indication or action regarded as a
   foreshowing; a foreboding; a presage; an augury.
   [1913 Webster]

         Bid go with evil omen, and the brand
         Of infamy upon my name.                  --Milton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Omen \O"men\, v. t. [imp. & p. p. Omened; p. pr. & vb. n.
   Omening.]
   To divine or to foreshow by signs or portents; to have omens
   or premonitions regarding; to predict; to augur; as, to omen
   ill of an enterprise.
   [1913 Webster]

         The yet unknown verdict, of which, however, all omened
         the tragical contents.                   --Sir W.
                                                  Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

omen
    n 1: a sign of something about to happen; "he looked for an omen
         before going into battle" [syn: omen, portent,
         presage, prognostic, prognostication, prodigy]
    v 1: indicate by signs; "These signs bode bad news" [syn:
         bode, portend, auspicate, prognosticate, omen,
         presage, betoken, foreshadow, augur, foretell,
         prefigure, forecast, predict]

The Devil's Dictionary (1881-1906):

OMEN, n.  A sign that something will happen if nothing happens.