ominous vs threatening

ominous and threatening both are adjectives.

Word	Noun	Adjective	Verb	Adverb
ominous	No	Yes	No	No
threatening	No	Yes	No	No

As adjectives, ominous and threatening are synonyms defined as:

ominous and threatening: threatening or foreshadowing evil or tragic developments

Other synonyms of ominous include baleful, forbidding, menacing, minacious, minatory, sinister.

ominous (adjective)	threatening (adjective)
threatening or foreshadowing evil or tragic developments	threatening or foreshadowing evil or tragic developments
presaging ill fortune	darkened by clouds