After Link wakes up on Mercay Island's beach at the beginning of Phantom Hourglass, the fairy Ciela brings him to see Oshus. Oshus warns link of the dangers the Ghost Ship and Temple of the Ocean King both face, and after witnessing Link's resolve to save Tetra, he reluctantly agrees to give Link a sword, as well as teaching him how to use it.
Towards the end of the game, it is revealed that Oshus is in fact the Ocean King himself, weakened by Bellum's curse. Once Link collects all three rare metals and Zauz has forged the Phantom Sword's blade, Oshus is able to combine the blade with the Phantom Hourglass, giving Link the power to defeat Phantoms. After Link defeats Bellum in the game's final boss battle, Oshus is returned to his true form.