The vision of Obadiah is a message from the Lord GOD about which nation?

Answer: Edom

Obadiah's vision is what the Lord GOD says about Edom; verse 8 speaks of destroying the wise men of Edom in the mountains of 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)