The town was first mentioned in 1108, at that time known under the name of Aldenburg. It became important due to its location at a ford of the navigable Hunte river. Oldenburg became a small county in the shadow of the much more powerful Hanseatic city of Bremen.
In 1448 the count of Oldenburg became king of Denmark under the name Christian I. Although far away from the Danish borders, Oldenburg was now a Danish exclave. The control over the town was left to the king's brothers, who established a short reign of tyranny.
The heyday of the town came with the rule of count Anton-Günther (ruled 1603-1667), who managed to keep Oldenburg out of the Thirty Years' War (1618-48) by donating valuable horses to the warlords. In 1607 he erected a Renaissance castle. Oldenburg was a wealthy town in a time of war and terror; its population and power considerably grew. After the death of Anton-Günther Oldenburg fell again under Danish authority. In 1667 the town was struck by a disastrous plague epidemic, and shortly after a fire destroyed Oldenburg. The Danish kings were not much interested in helping the town, so that it lost its importance completely.
In 1773 the Danish rule ended, and Oldenburg became a duchy. It was only now, that the destroyed buildings were rebuilt in a Classicist style. In 1893 a canal connecting the Hunte and the Ems rivers was finished, hence connecting the port of Oldenburg with the North Sea. Now the town gained economic importance. It remained a duchy until 1918.
Oldenburg is the birthplace of the philosopher Karl Jaspers. Its university is named after Carl von Ossietzky, who was awarded the Nobel Peace Prize in 1935 but was forbidden by the Nazis to accept it.