onset vs onrush

onset and onrush both are nouns.

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

As nouns, onset and onrush are synonyms defined as:

onset and onrush: (military) an offensive against an enemy (using weapons)

Other synonyms of onset include attack, onslaught.

onset (noun)	onrush (noun)
(military) an offensive against an enemy (using weapons)	(military) an offensive against an enemy (using weapons)
the beginning or early stages	a forceful forward rush or flow