This keyblade is given to you by Kari. In order to obtain it, you must finish your first trip to Hollow Bastion by beating Riku for the second time in the Grand Hall at the top of the castle. After this fight, you are turned into a heartless, and must run to the entry way of the castle to re-unite with Kari, Donald, & Goofy. After, you are sent to Traverse town.
When you arrive in Traverse Town, go speak with Cid outside the Accessory/Synthesis shop. He will tell you he hid the Navigation Gummi you need in the secret waterway. Go here, and talk to Kari. She brings your attention to a bright light on the wall. Run there to get the Navi-Gummi. After the cutscene, talk to Kari again to get the Oathkeeper Keyblade.