The difference between Antagonist and Opponent

When used as nouns, antagonist means an opponent or enemy, whereas opponent means one who opposes another.

Opponent is also adjective with the meaning: opposing.

check bellow for the other definitions of Antagonist and Opponent

Antagonist as a noun:

An opponent or enemy.
Antagonist as a noun:

One who antagonizes or stirs.
Antagonist as a noun (biochemistry):

A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
Antagonist as a noun (authorship):

The main character or force opposing the protagonist in a literary work or drama.
Antagonist as a noun (anatomy):

A muscle that acts in opposition to another.

Examples:

"A flexor, which bends a part, is the antagonist of an extensor, which extends it."

Opponent as a noun (historical):

One who opposes another; one who works or takes a position against someone or something; one who attempts to stop the progress of someone or something. One who opposes another physically (in a fight, sport, game, or competition). One who opposes another in words (in a dispute, argument or controversy). One who is opposed or objects (to a policy, course of action or set of ideas). The participant who opens an academic debate by putting forward objections to a theological or philosophical thesis.

Examples:

"The person who ran against her in the last election proved to be a formidable opponent."

"During the crackdown, many opponents of the regime were arrested."

"She was a dedicated opponent of the death penalty."

Opponent as an adjective (obsolete):

Opposing; adverse; antagonistic.
Opponent as an adjective (obsolete):

Situated in front; opposite.