When Jack realized Oogie Boogie has returned, he tells Sally about the evil.

Jack Skellington: LISTEN, SALLY! Oogie has returned!

Sally: Really?!

Jack Skellington: Yes, it's the truth! An evil named Set has managed to resurrect him!

Sally: Oogie... A familiar enemy of ours...

Meanwhile, at the Halloween Town Square...

Mayor: It is with great distress that evil has returned in the form of Oogie Boogie, our nemesis!

All: (gasp)

Child: Not Oogie Boogie! He's so mean!

Dr. Finkelstein: Yes, AND IT'S THE OLD TRUTH! And I have proof! Read it.

Clown: Very evil indeed. What should we do now?

Jack Skellington: We must head to Christmas Town, where we have the Conference of the Seven Kings. COME ON!

Meanwhile, Jack returns to Christmas Town, where Santy Claus welcomes him again.

Santy Claus: Seasons Greetings, Jack! How are you?

Jack Skellington: Fine, Claus.

Five minutes later in the conference...

Santy Claus: It's true that Oogie was resurrected.