Duchess Therese Petrovna of Oldenburg
Duchess Therese Wilhelmine Olga Friederike of Oldenburg (30 March 1852 – 19 April 1883) was the youngest daughter of Duke Peter Georgievich of Oldenburg and his wife Princess Therese of Nassau-Weilburg.
Duchess Frederica of Württemberg
Duchess Frederica of Württemberg (27 July 1765 – 24 November 1785) was a daughter of Frederick II Eugene, Duke of Württemberg and Friederike Dorothea of Brandenburg-Schwedt.
Duke George of Oldenburg
Duke Peter Frederick George of Oldenburg (9 May 1784 – 27 December 1812) was a younger son of Peter I, Grand Duke of Oldenburg and his wife Duchess Frederica of Württemberg.
He was a son-in-law of Paul I of Russia through marriage to his daughter Grand Duchess Catherine Pavlovna of Russia.
He was referred to as a prince in Russia, Prince Georgy Petrovich Oldenburgsky.
Christian, Duke of Oldenburg
Christian, Duke of Oldenburg( born 1 February 1955) is the head of the Grand Ducal Family of Oldenburg.
Anthony I, Count of Oldenburg
Anthony I, Count of Oldenburg and Delmenhorst( 1505 – 22 January 1573, Oldenburg) was a member of the House of Oldenburg and was the Imperial Count of the Counties of Oldenburg and Delmenhorst within the Holy Roman Empire of German Nation.
His parents were John V, Count of Oldenburg( 1460- 1525) and Anna of Anhalt- Zerbst.
Conrad I, Count of Oldenburg
Conrad I of Oldenburg (died 1347) was the Count of Oldenburg from 1344 to 1347.
He succeeded his brother, John III of Oldenburg.
He was the son of John II of Oldenburg and Hedwig of Diepholz.
Conrad married Ingeborg, the daughter of Count Gerhard IV of Holstein-Plön.
They had four children:
Conrad I of Oldenburg is also a male-line ancestor of Queen Margrethe II of Denmark, King Harald V of Norway, and of Philip, Duke of Edinburgh, and thus of Charles, Prince of Wales, and Prince William, Duke of Cambridge.
John VII, Count of Oldenburg
Count John VII of Oldenburg and Delmenhorst( nicknamed" the Dike Builder"; 9 September 1540 in Oldenburg – 12 November 1603 in Oldenburg) was a member of the House of Oldenburg and was the ruling Count of County of Oldenburg from 1573 until his death.
His parents were Count Anthony I of Oldenburg and Sophie of Saxe- Lauenburg.
Duke Peter of Oldenburg
Duke Constantine Frederick Peter 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 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.
Peter of Oldenburg was a scholar and philanthropist.
He was also noted composer of music.
In 1857 he composed the music for Marius Petipa's ballet" La Rose, la Violette et le Papillon".
The" Pas d' Esclave" from the ballet" Le Corsaire", which is taken from his score for this work, is still heard in theatres all over the world.
Duchess Eilika of Oldenburg
Duchess Eilika of Oldenburg( née Duchess Eilika Helene Jutta Clementine of Oldenburg; born 22 August 1972) is the wife of Georg von Habsburg, the third in the line of succession to the former Austro- Hungarian throne.
She was in line of succession to the British throne before her marriage to a Roman Catholic.
She is a daughter of Duke Johann of Oldenburg and his wife Countess Ilka of Ortenburg.
Her father is a younger son of Nikolaus, Hereditary Grand Duke of Oldenburg and his wife Princess Helena of Waldeck and Pyrmont.
John V, Count of Oldenburg
John V, Count of Oldenburg and Delmenhorst (also counted as John XIV, including also non-ruling namesake siblings; 1460, Oldenburg – 10 February 1526, Oldenburg) was a member of the House of Oldenburg.
He was the ruling Count of Oldenburg from 1500 to 1526.
His parents were Gerhard VI, Count of Oldenburg and Adelheid of Tecklenburg.

Question: Who is the maternal grandfather of Duke George Of Oldenburg?