bombing vs onset

bombing and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
bombing	Yes	No	No	No
onset	Yes	No	No	No

As nouns, onset is a hypernym of bombing; that is, onset is a word with a broader meaning than bombing:

Other hypernyms of bombing include attack, onrush, onslaught.

bombing (noun)	onset (noun)
an attack by dropping bombs	(military) an offensive against an enemy (using weapons)
the use of bombs for sabotage; a tactic frequently used by terrorists	the beginning or early stages