Forsaken hunters automatically know how to tame undead creatures. Hunters of other races must learn the skill from a Simple Tome of Bone-Binding which is a rare drop from miscellaneous Elite NPCs throughout Maldraxxus and its associated dungeons: Plaguefall and the Theater of Pain.
Location & Notes:
Located in Mandokir's Domain, Zul'Gurub (Dungeon). Ohgan is part of the Bloodlord Mandokir encounter and initially serves as the boss's mount. To tame him you'll have to wait until Mandokir dismounts and Ohgan kills all of the Chained Spirits that surround the arena; before then he refuses to be distracted by you.