Against which nation is the prophecy of Obadiah mainly directed?

Answer: Edom

Obadiah's vision is "what the Lord GOD says about Edom," announcing judgment on the nation descended from Esau.

“This is the vision of Obadiah: This is what the Lord GOD says about Edom—We have heard a message from the LORD; an envoy has been sent among the nations to say, “Rise up, and let us go to battle against her!”—”— Obadiah 1:1 (BSB)