Here are links for each of the OTG guilds. So it would depend upon which guild you are interested in.
Alliance OTG guild: WoW has expanded guild functionality over time, allowing you to join OTG from any server and any faction.
Horde OTG guild:
Note: Hashberry noted that the Horde guild invites work if you’re on either Kul Tiras or Bladefist, since those servers are merged together. I think the majority are on Kul Tiras.