Short answers, pulled from the story.
The county of Oldenburg was raised to the rank of a duchy in 1777, four years after the 1773 Treaty of Tsarskoye Selo transferred it to Frederick August of the Holstein-Gottorp branch. The duchy existed until 1810, when it was annexed into the French Empire.
Napoleon's French Empire annexed the Duchy of Oldenburg in 1810. The annexation was one of the causes of a diplomatic rift between France and Russia, contributing to the conflict that led to war in 1812 and ultimately to Napoleon's downfall. The duchy was restored as a grand duchy in 1815 after Napoleon's defeat.
The House of Oldenburg spread across the thrones of Denmark, Norway, Sweden, Greece, and Russia. Through Prince Philip, Duke of Edinburgh, a descendant of a junior branch of the Greek line of the house, the thrones of the United Kingdom and the other Commonwealth realms have been held since the passing of Queen Elizabeth II.
Anthony Gunther (1583-1667) ruled Oldenburg from 1603 and expanded the county by adding Kniphausen and Varel in 1624 and permanently uniting Delmenhorst in 1647. He kept Oldenburg neutral during the Thirty Years' War by donating horses to the Count of Tilly, protecting the county from widespread devastation. He also secured from the emperor the right to levy tolls on ships passing along the Weser River.
Jewish communities in Oldenburg date to the Middle Ages. The total Jewish population in the duchy's territory reached 1,359 in 1900 but fell to 1,015 by 1925 and to only 279 by 1933. Most were annihilated during the Holocaust, though some survived and returned after the war. The duchy was also the last part of Napoleon's conquered lands to implement his 1808 decree requiring Jews to adopt surnames.
Christian, son of Count Dietrich called Fortunatus, became Count of Oldenburg in 1440. In 1448 he was elected King of Denmark as Christian I, partly based on his maternal descent from earlier Danish kings. He also became King of Norway in 1450 and King of Sweden in 1457, making Oldenburg a Danish exclave.