opposer vs opposition

opposer and opposition both are nouns.

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

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

opposer: someone who offers opposition
opposition: an armed adversary (especially a member of an opposing military force)

Other hyponyms of opposer include dueler, duelist, dueller, duellist, enemy, foe, foeman, Luddite, withstander.

opposer (noun)	opposition (noun)
someone who offers opposition	the act of hostile groups opposing each other
	the action of opposing something that you disapprove or disagree with
	a body of people united in opposing something
	an armed adversary (especially a member of an opposing military force)
	a contestant that you are matched against
	a direction opposite to another
	the relation between opposed entities