directly vs at once

directly and at once both are adverbs.

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

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

directly and at once: without delay or hesitation; with no time intervening

Other synonyms of directly include forthwith, immediately, instantly, like a shot, now, right away, straight off, straightaway.

directly (adverb)	at once (adverb)
without deviation	without delay or hesitation; with no time intervening
in a forthright manner; candidly or frankly	simultaneously
without anyone or anything intervening
without delay or hesitation; with no time intervening