straight off vs straightaway

straight off is not a noun while straightaway is a noun.

Word	Noun	Adjective	Verb	Adverb
straight off	No	No	No	Yes
straightaway	Yes	Yes	No	Yes

As adverbs, straight off and straightaway are synonyms defined as:

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

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

straight off (noun)	straightaway (noun)
	a straight segment of a roadway or racecourse

straight off (adjective)	straightaway (adjective)
	performed with little or no delay

straight off (adverb)	straightaway (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening