Oslo's history dates back to around the year 1000. Since the Middle Ages, the city has undergone major changes and also changed its name several times.
The city's original name was Oslo. In the Middle Ages, it was located on the eastern bank of Bjørvika. After a dramatic fire in 1624, King Christian IV decided that the city should be rebuilt in the area just inside Akershus, and that it should be called Christiania. From 1877, the name was changed to Kristiania, and from 1925 the city was given back its original name, Oslo. For more than 1,000 years, the city has played an important role. Here you can read more about Oslo's history.
Source: Visit Oslo