former vs onetime

former is a noun but onetime is not a noun.

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

As adjectives, former and onetime are synonyms defined as:

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

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

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