The Oasis is a small area in Baldur's Gate II: Throne of Bhaal. There is nothing of real note in the area.
As soon as you arrive in the area, Jamis Tombelthen will speak with you and blame you for the destruction of Saradush. He and a large army of Tethyrian soldiers across the map will attack you. A good way to get rid of the soldiers is to cast Gate and summon a pit fiend. Jamis himself drops The Answerer.