OO = Only Olthoi can enter this portal. Specifically, this refers to the portals actually on Olthoi Island. (IE: As non-olthoi, you can go from Moarsman City to Olthoi Island, but not vice versa.)
Portal routes work both ways so taking the Olthoi Island portal @ 63.5N 81.9E gets you to Arwic
For a comprehensive in-game travel guide, try Connor's Olthoi Travel Journal (updated 12/18/2010). The guide utilizes the portals around the destination areas of the Olthoi Tunnels to maximize an Olthoi's travel options... even includes a way to get to the graveyard as an Olthoi!