death knell vs omen

death knell and omen both are nouns.

Word	Noun	Adjective	Verb	Adverb
death knell	Yes	No	No	No
omen	Yes	No	Yes	No

As nouns, omen is a hypernym of death knell; that is, omen is a word with a broader meaning than death knell:

Other hypernyms of death knell include portent, presage, prodigy, prognostic, prognostication.

death knell (noun)	omen (noun)
a bell rung to announce a death	a sign of something about to happen
an omen of death or destruction

death knell (verb)	omen (verb)
	indicate, as with a sign or an omen