Built above the ruins of Naniwa Palace (c 650), visible through the ground floor, this museum tells Osaka's story from the era of this early palace to the early 20th century. English explanations are pretty sparse, though much of the displays are highly visible, including a walk-through recreation of old city life. You can also rent an English-language audio guide (¥200). There are great views of Osaka-jō from the 10th floor.
The museum is just southwest of the castle park, in a sail-shaped building adjoining the NHK Broadcast Center.