now vs at once

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

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

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

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

now (adverb)	at once (adverb)
in these times	without delay or hesitation; with no time intervening
without delay or hesitation; with no time intervening	simultaneously
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