Dietrich of Oldenburg was the grandson of Ingeborg of Itzehoe, a Holstein princess who had married Count Conrad I of Oldenburg. After the death of her only brother, Count Gerhard V of Holstein-Itzehoe-Plön in 1350, Ingeborg and her issue were the heirs of her grandmother Ingeborg of Sweden (d. "ca." 1290, the first wife of Gerhard II of Holstein-Plön), the eldest daughter of King Valdemar of Sweden and Queen Sophia, who herself was the eldest daughter of King Eric IV of Denmark and his wife Jutta of Saxony who had no male descendants. Since there were no other living legitimate descendants of King Valdemar by this time, Dietrich was considered the heir general of Kings Valdemar I of Sweden and Eric IV of Denmark.