1. While in TB, go to the cavern that you normally spawn in when entering a portal in dalaran, shattrath or mage´s.
2. Go through the portal that leads to the dark portal.
3. Go through the dark portal
4. After arriving on the other side, turn right and jump to the orcs you see left hand the flightmaster and right hand a portal that leads to OG. Enter it.
5. Enjoy.
This is usefull when you are sitting in TB and want to OG obviously but need your hearthstone later or you have cd. This is way faster than flying and you avoid dalaran.