at once vs straightaway

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

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

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

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

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

at once (noun)	straightaway (noun)
	a straight segment of a roadway or racecourse

at once (adjective)	straightaway (adjective)
	performed with little or no delay

at once (adverb)	straightaway (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening
simultaneously