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.
Christian, Duke of Oldenburg
Christian, Duke of Oldenburg( born 1 February 1955) is the head of the Grand Ducal Family of Oldenburg.
Anne-Marie von Schutzbar genannt Milchling
Anne-Marie von Schutzbar genannt Milchling (3 July 1903 –1 January 1991) was the second wife of Nikolaus, Hereditary Grand Duke of Oldenburg, pretender to the Grand Duchy of Oldenburg.
Princess Helena of Waldeck and Pyrmont (1899–1948)
Princess Helena of Waldeck and Pyrmont( 22 December 189918 February 1948) was the only daughter of Friedrich, Prince of Waldeck and Pyrmont, last reigning Prince of Waldeck and Pyrmont, and wife of Nikolaus, Hereditary Grand Duke of Oldenburg.
Stéphanie, Hereditary Grand Duchess of Luxembourg
Stéphanie, Hereditary Grand Duchess of Luxembourg( born Countess Stéphanie Marie Claudine Christine de Lannoy on 18 February 1984), is the wife of Hereditary Grand Duke Guillaume, heir apparent to the throne of Luxembourg.
She became engaged to the Hereditary Grand Duke on 26 April 2012 and married him, in a civil ceremony, on 19 October 2012, followed by a religious service the next day.
Nikolaus, Hereditary Grand Duke of Oldenburg
Nikolaus, Hereditary Grand Duke of Oldenburg (10 August 18973 April 1970) was the eldest son of Frederick Augustus II, Grand Duke of Oldenburg, who was the last ruling Grand Duke of Oldenburg.
In 1931, Nikolaus succeeded to his father's titles and assumed the role of pretender to the Grand Duchy, until his death in 1970.
Charles Augustus, Hereditary Grand Duke of Saxe-Weimar-Eisenach (1844–1894)
Karl August, Hereditary Grand Duke of Saxe- Weimar- Eisenach( 31 July 1844 – 20 November 1894) was a German prince and Hereditary Grand Duke(" Erbgroßherzog") of Saxe- Weimar- Eisenach.
Duke Friedrich August of Oldenburg
Duke Friedrich August of Oldenburg( 11 January 1936- 9 July 2017) was a member of the House of Holstein- Gottorp.
He is a son of Nikolaus, Hereditary Grand Duke of Oldenburg and Princess Helena of Waldeck and Pyrmont.
Frederick Augustus II, Grand Duke of Oldenburg
Frederick Augustus II (16 November 1852 in Oldenburg – 24 February 1931 in Rastede) was the last ruling Grand Duke of Oldenburg.
He married Princess Elisabeth Anna of Prussia, daughter of Princess Maria Anna of Anhalt-Dessau and Prince Frederick Charles of Prussia.
After her death, he married Elisabeth Alexandrine of Mecklenburg-Schwerin.
Anton-Günther, Duke of Oldenburg
Anton- Günther, Duke of Oldenburg"( Anton- Günther Friedrich August Josias von Holstein- Gottorp, Erbgroßherzog von Oldenburg, Prinz von Lübeck,;" 16 January 1923 – 20 September 2014) was the head of the Grand Ducal Family of Oldenburg.
He was born in Lensahn the son of Hereditary Grand Duke Nikolaus of Oldenburg( 1897 – 1970) and his first wife Princess Helena of Waldeck and Pyrmont( 1899 – 1948).
Duke Anton- Günther was a grandson of Frederick Augustus II, the last Grand Duke of Oldenburg, and through his mother a grandson of Friedrich, the last Prince of Waldeck and Pyrmont.
He studied for a Diplom in Forestry and succeeded as head of the Grand Ducal Family of Oldenburg upon the death of his father, the Hereditary Grand Duke, on 3 April 1970.
He died in September 2014 aged 91.

Question: When did Nikolaus, Hereditary Grand Duke Of Oldenburg's father die?