Otto Heinrich Frank  (12 May 1889 – 19 August 1980) was a German-born Swiss businessman. He was the father of diarist Anne Frank and Margot Frank. As the sole member of his family to survive the Holocaust, he inherited Anne's manuscripts after her death, arranged for the publication of her diary as The Diary of a Young Girl in 1947, and oversaw its transition to the stage and screen.
Otto Frank was born in to a Jewish family. He was the second son of Alice Betty (née Stern) and Michael Frank.  His siblings were Robert Frank, Herbert Frank and Helene (Leni) Frank.  Otto was a cousin of the well known furniture designer Jean-Michel Frank, and a grandson of Zacharias Frank. He studied economics in Heidelberg from 1908 to 1909 and had a work experience placement at Macy’s Department Store in New York. 
World War I
Frank served in the Imperial German Army during the First World War. He was called up for military service in August 1915 and after training at a depot in Mainz he served in an artillery unit on the Western Front in which most soldiers were mathematicians and surveyors. He was attached to the infantry as a range-finder at the Battle of the Somme in 1916 and in 1917 he was promoted in the field to lieutenant and then served at the Battle of Cambrai. 
Marriage and children
He worked in the bank his family ran until it collapsed in the early 1930s. He married Edith Holländer —an heiress to a scrap-metal and industrial-supply business—on his birthday 12 May 1925 in Frankfurt. Their elder daughter, Margot, was born on 16 February 1926, followed by their younger daughter Anne on 12 June 1929.
World War II
As the tide of Nazism rose in Germany and anti-Jewish decrees encouraged attacks on Jewish individuals and families, Otto decided to evacuate his family to the safer western nations of Europe. In August 1933 he moved his family to Aachen, where his wife's mother resided, in preparation for a subsequent and final move to Amsterdam in the Netherlands. There he started a company, Opekta, that sold spices and pectin for use in the manufacture of jam. After Germany invaded the Netherlands in May 1940, Otto made his business look "Aryan" by transferring control to non-Jews.
In 1938 and 1941, Frank attempted to obtain visas for his family to emigrate to the United States or Cuba. He was granted a single visa for himself to Cuba on 1 December 1941, but it is not known if it ever reached him. Ten days later, when Nazi Germany and Fascist Italy declared war on the United States, the visa was cancelled.  
Otto Frank took his family into hiding on 6 July 1942 at the age of 53, in the upper rear rooms of the Opekta premises on the Prinsengracht. They were joined a week later by Hermann van Pels, who was known as Herman van Daan in Anne's diary, and his wife and son, and in November by Fritz Pfeffer, also known in Anne's diary as Mr. Dussel. Their concealment was aided by Otto Frank's colleagues Johannes Kleiman, whom he had known since 1923, Miep Gies, Victor Kugler, and Bep Voskuijl.
They were concealed for two years, until they were betrayed by an anonymous informant in August 1944. Frank, his family, the four people he hid with, and Kugler and Kleiman were arrested by SS Officer Karl Silberbauer. After being imprisoned in Amsterdam, the Jewish prisoners were sent to the Dutch transit camp of Westerbork and finally to Auschwitz Birkenau. During his time at Auschwitz, Otto Frank wrote to his mother in Switzerland. She had fled there in 1933 after Hitler came to power.   It was at Auschwitz, in September, that Frank was separated forever from his wife and daughters. He was sent to the men's barracks and found himself in the sick barracks when he was liberated by Soviet troops on 27 January 1945. He travelled back to the Netherlands over the next six months and set about tracing his arrested family and friends. By the end of 1945, he knew he was the sole survivor of the family, and of those who had hidden in the house on the Prinsengracht.
After Anne Frank's death was confirmed in the summer of 1945, her diary and papers were given to Otto Frank by Miep Gies, who had rescued them from the ransacked hiding place. Frank left them unread for some time but eventually began transcribing them from Dutch for his relatives in Switzerland. He was persuaded that Anne's writing shed light into the experiences of many of those who suffered persecution under Nazis and was urged to consider publishing it. He typed out the diary papers into a single manuscript and edited out sections he thought too personal to his family or too mundane to be of interest to the general reader. The manuscript was read by Dutch historian Jan Romein, who reviewed it on 3 April 1946, for the Het Parool newspaper. This attracted the interest of Amsterdam's Contact Publishing, and, in the summer of 1946, they accepted it for publication.
On 25 June 1947, the first Dutch edition of the diary was issued under the title Het Achterhuis (meaning: "The [Secret] Annex"). Its success led to an English translation in 1952, which subsequently led to a theatrical dramatisation and a cinematic version.
Otto Frank married a former neighbor from Amsterdam  and fellow Auschwitz survivor, Elfriede Geiringer (1905–1998), in Amsterdam on 10 November 1953, and both moved to Basel, Switzerland, where he had family, including relatives' children, with whom he shared his experiences.
In response to a demolition order placed on the building in which Otto Frank and his family had hidden during the war, he and Johannes Kleiman helped establish the Anne Frank Foundation on 3 May 1957, with the principal aim of saving and restoring the building, to allow it to be opened to the general public. With the aid of public donations, the building (and its adjacent neighbour) was purchased by the Foundation. It opened as a museum (the Anne Frank House ) on 3 May 1960, and it can still be visited today.