onetime vs former

onetime is not a noun while former is a noun.

Word	Noun	Adjective	Verb	Adverb
onetime	No	Yes	No	No
former	Yes	Yes	No	No

As adjectives, onetime and former are synonyms defined as:

Other synonyms of onetime include erstwhile, old, one-time, quondam, sometime.

onetime (noun)	former (noun)
	the first of two or the first mentioned of two

onetime (adjective)	former (adjective)
belonging to some prior time	referring to the first of two things or persons mentioned (or the earlier one or ones of several)
	belonging to the distant past
	belonging to some prior time
	(used especially of persons) of the immediate past