onetime vs old

onetime is not a noun while old is a noun.

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

As adjectives, onetime and old are synonyms defined as:

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

onetime (noun)	old (noun)
	past times

onetime (adjective)	old (adjective)
belonging to some prior time	of long duration; not new
	(used especially of persons) having lived for a relatively long time or attained a specific age
	skilled through long experience
	(used for emphasis) very familiar
	excellent
	just preceding something else in time or order
	belonging to some prior time