The earliest people the Romans recorded living in this region were the Chauci, a Teutonic tribe later absorbed into the Frisians. Medieval chroniclers liked to claim the counts of Oldenburg descended from Widukind, the Saxon chieftain who fought Charlemagne to a standstill, but the first verifiable count is Huno of Rustringen, who died in 1088 after founding the monastery at nearby Rastede. His descendants served as vassals of the dukes of Saxony, occasionally in rebellion, and rose to imperial-prince status when the Emperor Frederick I dismembered the Saxon duchy in 1189. From then on the family had a foothold among the German nobility, but a small one. Oldenburg in the high Middle Ages was a marshland county with a borrowed identity, frequently warred over by the bishop of Munster and the Hanseatic city of Bremen.

The northern and western reaches of what later became the grand duchy belonged to independent Frisian princes, many still pagan into the 13th century. The counts spent decades pushing into Frisian lands, claiming villages one by one and grinding the territory outward. The county of Delmenhorst sometimes detached as an appanage for younger branches of the family, in three separate stretches between 1266 and 1617. Oldenburg's heritage is one of slow, stubborn medieval accretion, less a great inheritance than a frontier earned in mud.

In 1448 King Christopher III of Denmark died at thirty-one without an heir. The Kalmar Union required Denmark, Norway and Sweden to choose a successor together, but Swedish-Danish tensions made joint negotiations impossible. When the Swedes crowned Karl Knutsson on their own, the Danish Privy Council scrambled for an alternative. Their first choice, Duke Adolf VIII of Schleswig-Holstein, declined and suggested his nephew Christian, the young Count of Oldenburg who had grown up at his court. The Danes elected him in September. He became King of Norway in 1449, then King of Sweden in 1457. Oldenburg suddenly belonged to a king. Christian governed Denmark from Copenhagen and left the town to his brothers Gerhard and Moritz, who promptly turned it into a small tyranny.

In 1460 Christian had to renounce his claim to Oldenburg as part of the Treaty of Ribe, which made him Duke of Schleswig and Count of Holstein. He transferred Oldenburg to his brother Gerhard, who waged so many wars on Hanseatic shipping that the league branded him a pirate. He died on a pilgrimage in Spain. Protestantism arrived under Count Anton I in the 16th century, suppressing the monasteries while still managing to stay on good terms with the Catholic Emperor Charles V. His grandson Anton Gunther succeeded in 1603 and proved a remarkable diplomat. Through strict neutrality during the Thirty Years' War, and by sending the warlord Tilly a famous gift of valuable horses, he spared Oldenburg the devastation that flattened most of Germany. He even talked the emperor into giving him toll rights on Weser shipping, which made the county wealthy in a time of war. He built a Renaissance castle in 1607 that still anchors the city.

Anton Gunther had no legitimate children. When he died in 1667 the main line died with him, and the county passed jointly to King Frederick III of Denmark and the Duke of Holstein-Gottorp, with Anton's illegitimate son Anton von Aldenburg installed as governor. Danish officials moved into the residence after von Aldenburg's death in 1680, and Oldenburg became an outlying province governed from the German Chancellery in Copenhagen. The Danish century was brutal. Plague struck in 1667 and again in 1668. In 1676 a lightning strike set off a fire that destroyed 700 houses and left 3,000 people homeless. The Christmas Flood of 1717 drowned more than 4,000 people in the county and ruined the dykes; the New Year's Flood of 1720 finished off what little had been rebuilt. Oldenburg stayed Danish until 1773, when a tangled diplomatic exchange involving Catherine the Great handed it to a different branch of the House of Oldenburg, and the county was elevated to a duchy the following year. The medieval county was over. The ford in the Hunte had carried the town a long way.