Prince Oskar of Prussia
Prince Oskar Karl Gustav Adolf of Prussia ("Oskar Karl Gustav Adolf Prinz von Preußen"; 27 July 1888 – 27 January 1958) was the fifth son of Wilhelm II, German Emperor and Augusta Victoria of Schleswig-Holstein-Sonderburg-Augustenburg.
Prince Wilhelm-Karl of Prussia
Prince Wilhelm Karl of Prussia(" Wilhelm Karl Adalbert Erich Detloff"; 30 January 1922, in Potsdam – 9 April 2007, in Holzminden) was the third son of Prince Oskar of Prussia, and the last surviving grandson of Wilhelm II, the last German Emperor.
He was the thirty- sixth Master of Knights(" Herrenmeister") of the Protestant( and largely German) Order of Saint John( Bailiwick of Brandenburg), also known as" Der Johanniterorden".
Princess Calixta of Lippe
Princess "Calixta" Agnes Adelaide Irmgard Helene Caroline Elise Emma of Lippe (14 October 1895 - 15 December 1982) was the wife of Prince Waldemar of Prussia, eldest son of Prince Henry of Prussia.
Augusta Victoria of Schleswig-Holstein
Augusta Victoria of Schleswig-Holstein (Auguste Viktoria Friederike Luise Feodora Jenny; 22 October 1858 – 11 April 1921) was the last German empress and queen of Prussia by marriage to Wilhelm II, German Emperor.
Princess Charlotte of Prussia (1831–1855)
Princess Charlotte of Prussia( 21 June 1831 – 30 March 1855), was by birth a Princess of Prussia and member of the House of Hohenzollern and by marriage Hereditary Princess of Saxe- Meiningen.
She was the eldest child and daughter of Prince Albert of Prussia( younger son of Frederick William III of Prussia and Louise of Mecklenburg- Strelitz) and his first wife Princess Marianne of the Netherlands( daughter of William I of the Netherlands and Wilhelmine of Prussia).
Princess Herzeleide of Prussia
Princess Herzeleide-Ina-Marie Sophie Charlotte Else of Prussia (25 December 1918 – 22 March 1989) was a member of the deposed House of Hohenzollern.
She was the only daughter of Prince Oskar of Prussia (the second youngest son of Emperor Wilhelm II) and his initially morganatic wife, Countess Ina Marie von Bassewitz.
She is the mother of Ernst-Johann Biron, Prince of Courland, the current head of the House of Biron.
Princess Elisabeth of Prussia
Princess Elisabeth of Prussia( 18 June 1815 – 21 March 1885) was the second daughter of Prince Wilhelm of Prussia and Landgravine Marie Anna of Hesse- Homburg and a granddaughter of Frederick William II of Prussia.
Prince Philip, Duke of Edinburgh is her great- great- grandson.
Countess Ina Marie von Bassewitz
Princess Oskar of Prussia, Countess of Ruppin (27 January 1888 – 17 September 1973) was a German aristocrat and the wife of Prince Oskar of Prussia.
Ernst-Johann Biron, Prince of Courland
Ernst- Johann Karl Oskar Eitel- Friedrich
Peter Burchard, Prince Biron von Courland( born 6 August 1940) is the senior male- line descendant of Ernst Johann von Biron, and as such heir to the Duchy of Courland and Semigallia.
He was born in Berlin, the eldest son of Karl, Prince Biron of Courland( 1907- 1982), and Princess Herzeleide of Prussia( 1918- 1989), the daughter of Prince Oskar of Prussia( a son of Emperor William II) and Augusta Viktoria of Schleswig- Holstein.
Prince Ernst- Johann succeeded his father as head of the Biron family following his death on 28 February 1982.
He is also a Knight of Justice( and, since 2008, an honorary commander in the Commandery of Silesia) of the" JohanniterordenJohannine Order"), to which he was admitted in 1940 and whose grand master is his cousin Prince Oskar of Prussia( Prince Ernst- Johann's uncles and grandfather had headed the Order throughout most of the twentieth century).
He is in line to the British throne through his descent from Queen Victoria's daughter, Victoria, Princess Royal.
Princess Marianne of Prussia
Princess Marianne of Prussia( 23 August 19131 March 1983) was a member of the House of Hohenzollern, daughter of Prince Friedrich Wilhelm of Prussia.

Question: What is the date of birth of Prince Oskar Of Prussia's mother?
Answer: 22 October 1858