old vs one-time

old is a noun but one-time is not a noun.

Word	Noun	Adjective	Verb	Adverb
old	Yes	Yes	No	No
one-time	No	Yes	No	No

As adjectives, old and one-time are synonyms defined as:

Other synonyms of old include erstwhile, former, onetime, quondam, sometime.

old (noun)	one-time (noun)
past times

old (adjective)	one-time (adjective)
of long duration; not new	belonging to some prior time
(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