Question: The grandfather of  Duke Peter Alexandrovich was the Duke of what European dynasty of North German Origin

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House and Merit Order of Peter Frederick Louis: The House and Merit Order of Duke Peter Frederick Louis (German: "Haus und Verdienstorden von Herzog Peter Friedrich Ludwig") or proper German Oldenburg House and Merit Order of Duke Peter Frederick Louis (German: "Oldenburgische Haus- und Verdienstorden des Herzogs Peter Friedrich Ludwig") was a civil and military order of the Grand Duchy of Oldenburg, a member state of the German Empire.  The order was founded by Grand Duke Augustus of Oldenburg on November 27, 1838, to honor his father, Peter Frederick Louis of Oldenburg.  It became obsolete in 1918 after the abdication of the last grand duke.

Duke Peter Alexandrovich of Oldenburg: Peter Alexandrovich of Oldenburg (Peter Friedrich Georg, Duke of Oldenburg) (21 November 1868 – 11 March 1924) was the first husband of Grand Duchess Olga Alexandrovna of Russia, the youngest sister of Tsar Nicholas II.

Astur-Leonese dynasty: The Asturian or Astur-Leonese dynasty ("dinastía asturiana" or "astur-leonesa"), known in Arabic as the Beni Alfons or Banu Alfonso ("sons of Alfonso"), was the ruling family of Asturias, Galicia and León from about 740 until 1037.  The first king of the dynasty was Alfonso I, son of Duke Peter of Cantabria.  For the first century, rule alternated between Alfonso's descendants and those of his brother, Fruela of Cantabria.  With the death of Alfonso II a century later, a younger branch of the family took the throne, headed by Ramiro I.  For most of the 10th century, internecine squabbles resulted in divided rule and succession struggles among various branches of the family, which were brought to an end by the succession of Bermudo II in 984.  The rule of the dynasty ended with the defeat and death of Bermudo III in 1037, the throne passing via his sister to Ferdinand I of León.

North German basin: The North German Basin is a passive-active rift basin located in central and west Europe, lying within the southeastern most portions of the North Sea and the southwestern Baltic Sea and across terrestrial portions of northern Germany, Netherlands, and Poland.  The North German Basin is a sub-basin of the Southern Permian Basin, that accounts for a composite of intra-continental basins composed of Permian to Cenozoic sediments, which have accumulated to thicknesses around 10 - .  The complex evolution of the basin takes place from the Permian to the Cenozoic, and is largely influenced by multiple stages of rifting, subsidence, and salt tectonic events.  The North German Basin also accounts for a significant amount of Western Europe's natural gas resources, including one of the worlds largest natural gas reservoir, the Groningen gas field.

Wittrock: Wittrock is a surname of North German origin.  People with the surname include:

Frink (surname): Frink is a surname of North German origin.

Duke Peter Georgievich of Oldenburg: Duke Konstantin Friedrich Peter Georgievich of Oldenburg (26 August 1812 in Yaroslavl, Russian Empire – 14 May 1881 in St. Petersburg, Russian Empire) was a Duke of the House of Oldenburg.  He was the grandfather of Duke Peter Alexandrovich of Oldenburg as well as grandfather of Grand Duke Nicholas Nikolaevich, General of the Imperial Russian Army during World War I.  His great-great grandson, Nicholas Romanov, was the President of the Romanov Family Association until his death in 2014.

North German Confederation Treaty: The North German Confederation Treaty (in German "Augustbündnis", or Alliance of August) (also called the North German Federation Treaty and the Treaty of 18 August 1866) was the treaty between the Kingdom of Prussia and other northern and central German states that initially created the North German Confederation, which was the forerunner to the German Empire.  This treaty, and others that followed in September and October, are often described as the "August treaties", although not all of them were concluded in August 1866.

House of Oldenburg: The House of Oldenburg is a European dynasty of North German origin.  It is one of Europe's most influential royal houses, with branches that rule or have ruled in Denmark, Iceland, Greece, Norway, Russia, Sweden, Schleswig, Holstein, and Oldenburg.  The current Queen of Denmark and King of Norway, the former King of Greece, the consorts of Greece and the United Kingdom, as well as the first twelve names in the line of succession to the British throne, are all patrilineal members of the Glücksburg branch of this house.

David Petersen (composer): David Petersen (born Lübeck ca. 1650 or 1651 – died Amsterdam, before 5 May 1737) was a violinist and composer of north German origin active in the Netherlands (United Provences).

Supporting paragraphs: Duke Peter Georgievich of Oldenburg House of Oldenburg
Answer: House of Oldenburg