on the job vs working

on the job is not a noun while working is a noun.

Word	Noun	Adjective	Verb	Adverb
on the job	No	Yes	No	No
working	Yes	Yes	No	No

As adjectives, on the job and working are synonyms defined as:

on the job (noun)	working (noun)
	a mine or quarry that is being or has been worked

on the job (adjective)	working (adjective)
actively engaged in paid work	actively engaged in paid work
	(of e.g. a machine) performing or capable of performing
	adopted as a temporary basis for further work
	adequate for practical use; especially sufficient in strength or numbers to accomplish something
	serving to permit or facilitate further work or activity