working vs on the job

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

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

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

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

working (adjective)	on the job (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