at once vs straight off

at once and straight off both are adverbs.

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

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

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

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

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