concern vs onus

concern and onus both are nouns.

Word	Noun	Adjective	Verb	Adverb
concern	Yes	No	Yes	No
onus	Yes	No	No	No

As nouns, onus is a hyponym of concern; that is, onus is a word with a more specific, narrower meaning than concern:

Other hyponyms of concern include bugaboo, burden, encumbrance, incumbrance, load, business.

concern (noun)	onus (noun)
something that interests you because it is important or affects you	an onerous or difficult concern
something or someone that causes anxiety; a source of unhappiness
a feeling of sympathy for someone or something
an anxious feeling
a commercial or industrial enterprise and the people who constitute it

concern (verb)	onus (verb)
be relevant to
be on the mind of