*WARNING! THIS GLITCH MAY CAUSE YOU TO NOT BE ABLE TO EXIT THE BUILDING YOU ENTER!*
First, get yourself the Oni Link in the field, then return to Clock Town (take care of all other things for Oni Link BEFORE you become Oni Link, or it will not work, seeing as how you can't use items.) Now that you're Oni Link, you'll be wielding his fantastic Double Helix. From testing, we've found the safest place to do this is in the Milk Bar. When you enter the Milk Bar, weapons will be banned, causing Oni Link to be weaponless. Well, if you attempt to block, he'll use his right arm as a sheild! You can run around like this for a good amount of time, but actual field testing if the weapon works is impossible, seeing as how he has a weapon outside...
Also, while in a building, if you hold down Z, Link's sword will glow even though it's not there!
*AGAIN, WARNING! THE MILK BAR IS THE FIRST TO BE TESTED THAT ALLOWS YOU TO EXIT THE BUILDING! OTHER'S WILL GET ONI LINK STUCK!!*