work on vs overwork

work on is not a noun while overwork is a noun.

Word	Noun	Adjective	Verb	Adverb
work on	No	No	Yes	No
overwork	Yes	No	Yes	No

As verbs, overwork is a hyponym of work on; that is, overwork is a word with a more specific, narrower meaning than work on:

Other hyponyms of work on include hot-work, cold work, coldwork, make over, retread, rework, rack, tool, till.

work on (noun)	overwork (noun)
	the act of working too much or too long

work on (verb)	overwork (verb)
shape, form, or improve a material	use too much
to exert effort in order to do, make, or perform something	work excessively hard