antagonist vs opposer

antagonist and opposer both are nouns.

Word	Noun	Adjective	Verb	Adverb
antagonist	Yes	No	No	No
opposer	Yes	No	No	No

As nouns, antagonist and opposer are synonyms defined as:

Other synonyms of antagonist include adversary, opponent, resister.

antagonist (noun)	opposer (noun)
a drug that neutralizes or counteracts the effects of another drug	someone who offers opposition
a muscle that relaxes while another contracts
someone who offers opposition