Ocarina Items is the term used to describe a glitch which allows link to use and play other items as if they were the ocarina. This leads to the ability to play songs in places where the regular ocarina item is unusable, or in situations where the player does not have the regular ocarina (such as if it has been overwritten using bottle duplication over B button ).
Though the normal way you would do Ocarina Items in Majora's Mask 64 has been patched, there are still several ways to perform it or even bypass the patch.
Catch something in a bottle so you have a bottled content in hand, put on the Kamaro or Bremen mask, perform ESS, press the bottle button followed by the B button, Link should start playing an invisible Ocarina.
Have a bottled content in hand, perform Action Swap either by jumping on water or with crouching and do the same as Method 1, perform ESS, press the bottle button followed by the item you quickdrew and Link should start playing a fake ocarina (the item he has in hand).
Being on Goron form, catch something in a bottle so you have a bottled content in hand, perform ESS, press the bottle button followed by the B button, Link should throw a punch (if you're holding L) and then Link should start playing the drums.
Note that this method of Ocarina Items has been patched in the version 1.1 update.
You need at least 2 bombs to perform this trick.
Note: Link's arms should be coming down at the same time as the item is used. This is a 1 frame trick.
If performed correctly, Link's arms should come down and he should be playing an invisible ocarina.