Oath to Order

The "Oath to Order" is a song featured in Majora's Mask.

Learning Location
It is a mystical ballad taught to Link when he rescues the first of the Four Giants from Woodfall Temple during his quest to save Termina from Majora's Mask and the falling Moon.

Uses
Its function is to summon the Four Giants on the eve of the Carnival atop the Clock Tower. Once all of the Four Giants have been freed from their seal, they will come and prevent the evil menace from destroying Termina while Link and Tatl enter the moon to defeat the evil inside Majora's Mask.

In the case that not all the Giants have been saved, the ones who have been released from their seal will still come upon calling, but they will not be able to stop the Moon from crashing into Clock Town on their own.

Etymology
An oath is an appeal to a deity, or to some revered person or thing, to witness one's determination to speak the truth or to keep a promise. This fits with the Four Giants' promise to come and put an order to Termina whenever Link plays the melody.