35978d12_Zelda___Majora_s_Mask____Songs__Where_to_play
[How to play] Oath to Order [How to learn] By releasing the spirit from the evil clutches of Odolwa's mask, the spirit (One of the four giants, the things Tael was trying to tell you about) will teach you this song. Alls ya gotta do is defeat Odolwa in Woodfall temple and you've got this hip, new song in your inventory. [Where to play] 
You only have to play this song once. That is when you have freed all four spirits from their masky prisons and are on top of the clock tower after midnight on the final day. Play it after the cutscene with the skull kid and "the four who are there" will come to you and stop the moon from falling. ^_^
The Legend of Zelda : Majora's Mask -- Songs
Where to play