To get the phantom sword, you need to have collected three metals, and give them to Zauz. You get it by following the main storyline, and there is no way to get it before then.
After you beat the Temple of the Ocean King for the 5th time, you will get some crystals. Give them to Zauz the Blacksmith and go to the girl's ship. Fight her and go to Zauz's Island to see if he's...
You go to the end of the temple of the ocean king and kill 9 phantoms with the sword (By hitting them in their backs) and beat the final bosses 3 forms.
ok the Oshus Sword is in the Storage next to Oshus' house. Just break the barrels. the Phantom Sword is not until you get all the fairies then u collect pure metals to create it.