straightaway vs straight off

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

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

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

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

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

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

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

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