Omen adjectives are listed in this post. Each word below can often be found in front of the noun omen in the same sentence. This reference page can help answer the question what are some adjectives commonly used for describing OMEN.
auspicious, bad, best, better, dark
evil, fatal, favorable, favourable, fearful
first, fortunate, good, happy, ill
inauspicious, lucky, propitious, sad, sinister
sure, terrible, unfavorable, unfavourable, unlucky
Hope this word list had the adjective used with omen you were looking for. Additional describing words / adjectives that describe / adjectives of various nouns can be found in the other pages on this website.