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]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "omen":
   actuarial prediction, adumbrate, adumbration, apocalypse, augur,
   augury, auspicate, auspice, betoken, betokening, betokenment, bode,
   boding, danger sign, early symptom, falling barometer, forebode,
   foreboding, forecast, forecasting, foreshadow, foreshadowing,
   foreshow, foreshowing, foresight, foretelling, foretoken,
   foretokening, forewarning, gathering clouds, guesswork, harbinger,
   have a hunch, have an intimation, high sign, improbability,
   indicant, indication, portend, portent, precursor, predict,
   prediction, prefiguration, prefigurement, prefiguring,
   preindication, preliminary sign, premonition, premonitory shiver,
   premonitory sign, premonitory symptom, presage, presaging,
   presentiment, preshowing, presignifying, probability, prodroma,
   prodrome, prognosis, prognostic, prognostication, promise,
   prophecy, prophesying, prospectus, quarantine flag, red flag,
   red light, shadow, shadow forth, sign, skull and crossbones,
   soothsay, speculation, statistical prediction, storm petrel,
   stormy petrel, symptom, thundercloud, thunderhead, token, tokening,
   type, vaticination, warning sign, warning signal, yellow flag,
   yellow jack



The Devil's Dictionary (1881-1906):

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