Old Siwa Island (OSI) is the fifth dungeon in the game, and you do not get an automatic quest for it. The dungeon is located in Kaslow (X:277, Y:233). You will need items for this Main Quest, so make sure you have space/your sprite out.
Once you reach level 46, go to Kaslow and the quest [L45. The Call of the Past] will then automatically pop up for you. The quest requires you to talk to Drifter Weidor in Kaslow. Once you turn the quest in to him you'll have to talk to him again and select "Touch the Time Compass in Weidor's hand" to go in to the dungeon for the rest of the quests.
Talk to Village Head Aquil (X:175, Y:422) for [L40. Crisis of Siwa Island] which asks you to kill the first boss, Furuguru (X:303, Y: 424). You do not need an item or mob kills to complete this quest. Once you defeat Furuguru turn in the quest to Aquil and he'll give you another quest called [L48. Find Babama] He will also give you an item called Ruby Ring so make sure you have at least one backpack space or you won't be able to accept the quest.
Also talk to Little Boy Ludaumann to receive [L49. Shadow Beneath the Falls] which requires you to kill Vice-Chief Fishman and collect Giant Fishman Gills from him. Interestingly, Vice-Chief Fishman moves around and is not in just one set place. He can typically be found anywhere from (X:294, Y:161) to (X:372, Y: 172). Once done, turn it back in to Ludaumann for the title [Grandbaron Liberator] (STR +1)
Once you've finished [L48. Find Babama] and talk to Apprentice Sage Babama he'll give you [L50. The Grandbaron's Threat] which asks you to kill Ancient Grandbaron and collect Grandbaron's Weakened Soul from him. Once you've completed the quest, turn it in to Aquil.
This Main Quest is typically the same as the first four, save for the extra item you get from one of the NPCs.
You can either give the item (Ruby Ring) Aquil gave you to Apprentice Sage Babama by selecting "Lost Ring" in his chat menu, or you can keep it for yourself. If you give it to him however, he'll give you an item called Babama's Mace which you can wear or put in to your archive.
The Main Quest technically gives the title [Grandbaron Liberator], but the Mission Order gives the title [Time Traveler] (Evade +7%, Defense -5%). There is no "final boss room" to this dungeon, so you just need to kill the first three bosses for the Mission Order.