The County of Oldenburg is a county of the Holy Roman Empire.In 1448, Christian I of Denmark (of the House of Oldenburg), Count of Oldenburg becomes King of Denmark, and later King of Norway and King of Sweden.

One of his grandsons, Adolf, Duke of Holstein-Gottorp is the first Duke of Holstein-Gottorp.

=When the main lineage of the House of Oldenburg, i.e.

Anthony Günther, Count of Oldenburg dies in 1667, it falls to the Danish Royal Family.

Charles Frederick of the line of the Dukes of Holstein-Gottorp marries Grand Duchess Anna Petrovna of Russia, daughter of Peter the Great.

His first cousin, Frederick August I becomes Duke of Oldenburg in 1774.

One of his brothers, Adolf Frederick becomes King of Sweden.

Another brother, Prince Georg Ludwig of Holstein-Gottorp, is father of Peter I, who becomes Grand Duke of Oldenburg in 1823.

Subsequent Rulers of Oldenburg are all his descendants