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If you've only visited Pandaria by using warlock summons or mage portals, the portal will not appear. Check your quest log or an Orgrimmar Warchief's Command Board for The Art of War quest. Once the quest line takes you to Pandaria, the portal in Orgrimmar will open. You must be level 85 or higher to start this quest. If you've transferred from Alliance since your first visit to Pandaria, you'll have to repeat this quest from the Horde side. Start the quest chain with The Art of War quest, as described above. If you purchased a character boost to level 90 or above, a bug may mark the first quest complete. Fly to the airship to the northwest of the docks instead to find the next quest in the chain.
Confirm that you've completed the intro quest. Repeat the quest line after a faction change. Unlock the portal on a boosted character.