Synonyms for One by one:

adv.

apiece (adverb)
severally.
one at a time (adverb)
separately, one at a time, singly, individually.

adv

separately (adverb)
individually.

n.

one by one (noun)
on an individual basis, one at a time, severally, one after another, separately, individually, singly, by the piece.

Other synonyms:

Other relevant words:
individually.