withstander vs opposer

withstander and opposer both are nouns.

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

As nouns, opposer is a hypernym of withstander; that is, opposer is a word with a broader meaning than withstander:

Other hypernyms of withstander include adversary, antagonist, opponent, resister.

withstander (noun)	opposer (noun)
an opponent who resists with force or resolution	someone who offers opposition
a fighter who holds out against attack