Antagonist

The Suitors (especially Antinoös and Eurymachos)

Throughout the entire epic, the suitors are in the process of eating poor Penelope and Telemachos out of house and home. They transgress all traditional ideas of honor by proposing marriage to a woman still waiting and grieving for her husband. They don’t show any respect to the man of the house, Telemachos. They also beat up beggars.

Poseidon

Poseidon is a more conventional antagonist; he has a lot more power than the mortal Odysseus and uses it against him for purposes of vengeance (over that whole Cyclops thing). Poseidon is a more distant villain than the suitors, working his magic from afar and never confronting Odysseus face-to-face for any sort of showdown. Of course, this makes it difficult for Odysseus to best or beat Poseidon in any sort of traditional way; instead, he must make peace with his enemy after suffering the hardship of his punishment.