straight off vs at once

straight off and at once both are adverbs.

Word	Noun	Adjective	Verb	Adverb
straight off	No	No	No	Yes
at once	No	No	No	Yes

As adverbs, straight off and at once are synonyms defined as:

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

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

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