at once vs now

at once is not a noun while now is a noun.

Word	Noun	Adjective	Verb	Adverb
at once	No	No	No	Yes
now	Yes	No	No	Yes

As adverbs, at once and now are synonyms defined as:

at once (noun)	now (noun)
	the momentary present

at once (adverb)	now (adverb)
without delay or hesitation; with no time intervening	in these times
simultaneously	without delay or hesitation; with no time intervening
	used to preface a command or reproof or request
	at the present moment
	in the historical present; at this point in the narration of a series of past events
	in the immediate past
	(prefatory or transitional) indicates a change of subject or activity