Hello
FairyCrystal is right, they are really no set in stone rules, but it is generally accepted that energy flows in through the left side, and flows out through the right side... So in that belief you might want to wear it on your left side to sort of buffer you from surrounding energies. Just my humble opinion here, but over the years I have found that just bringing a crystal into your energy fields has an effect on them, whether its through a ring, pendant or carrying it in your pocket.
Enjoy! Obsidian is awesome!