opposition vs opposer

opposition and opposer both are nouns.

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

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

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

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

opposition (noun)	opposer (noun)
the act of hostile groups opposing each other	someone who offers opposition
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