Question: Who helped Hans-Ulrich von Oertzen develop Operation Valkyrie?

Context paragraphs:
Klaus von Oertzen: Baron Claus-Detlof von Oertzen (1894 – 1991) was involved in the motor industry for most of his long life and is sometimes referred to as the “Father of Volkswagen of South Africa”.

Hans-Ulrich von Oertzen: Born in Berlin to an aristocratic family, Oertzen followed his father into the army and trained as a general staff officer.  During the Second World War he was promoted to Major and by 1943 was serving in Army Group Centre under Generalmajor Henning von Tresckow, who was one of the leading members of the resistance.  He then helped Claus von Stauffenberg to develop Operation Valkyrie.  On 26 March 1944, he married Ingrid von Oertzen Langenn, and a selection of 240 letters he wrote to her from 1942 to 1944 were recently published in book form in Germany.

20 July plot: On 20 July 1944, Claus von Stauffenberg and other conspirators attempted to assassinate Adolf Hitler, Führer of Nazi Germany, inside his Wolf's Lair field headquarters near Rastenburg, East Prussia.  The name Operation Valkyrie, originally referring to part of the conspiracy, has become associated with the entire event.  The apparent aim of the assassination attempt was to wrest political control of Germany and its armed forces from the Nazi Party (including the SS) and to make peace with the western Allies as soon as possible.  The underlying desire of many of the high-ranking Wehrmacht officers involved was apparently to show the world that not all Germans were like Hitler and the Nazi Party.  The details of the conspirators' peace initiatives remain unknown, but they likely would have included unrealistic demands for the confirmation of Germany's extensive annexations of European territory.

Jahrbuch Medien und Geschichte: The Jahrbuch Medien und Geschichte (English: "Yearbook for Media and History") is an annual academic journal covering the history of mass media.  The journal is published in German and (fewer articles) in English.  It is the official journal of the "Studienkreis Rundfunk und Geschichte", a scientific association that was founded in 1968.  Every yearbook has a single, unified theme.  The contributions are not limited to the traditional aspects of radio and television research.  The first editors had an expanded definition of media.  The yearbook has been published from 2001 till 2005 by the UVK Verlagsgesellschaft and since 2011 by the Herbert von Halem Verlag.  The current editors are Christoph Rohde, Christian Schurig, and Hans-Ulrich Wagner.

Henning von Tresckow: Hermann Henning Karl Robert von Tresckow (10 January 1901 – 21 July 1944) was an officer in the German Army who helped organize German resistance against Adolf Hitler.  He attempted to assassinate Hitler on 13 March 1943 and drafted the Valkyrie plan for a coup against the German government.  He was described by the Gestapo as the "prime mover" and the "evil spirit" behind the plot of 20 July 1944 to assassinate Hitler.  He committed suicide at Królowy Most on the Eastern Front upon the plot's failure.

Claus von Stauffenberg: Claus Philipp Maria Schenk Graf von Stauffenberg (15 November 1907 – 21 July 1944) was a German army officer and member of the German nobility who was one of the leading members of the failed 20 July plot of 1944 to assassinate Adolf Hitler and remove the Nazi Party from power.  Along with Henning von Tresckow and Hans Oster, he was one of the central figures of the German Resistance movement within the "Wehrmacht".  For his involvement in the movement, he was executed by firing squad shortly after the failed attempt known as Operation "Valkyrie".

Jo Baier: Jo Baier (born 13 February 1949) is a German film director and writer.  He directed more than twenty films since 1982 and is known for "Operation Valkyrie" (2004), "Henri 4" (2010) and "Not All Were Murderers" (2006).  He is married to Gertrud Baier.

Valkyrie (film): Valkyrie is a 2008 American-German historical thriller film set in Nazi Germany during World War II.  The film depicts the 20 July plot in 1944 by German army officers to assassinate Adolf Hitler and to use the Operation Valkyrie national emergency plan to take control of the country.  "Valkyrie" was directed by Bryan Singer for the American studio United Artists, and the film stars Tom Cruise as Colonel Claus von Stauffenberg, one of the key plotters.  The cast included Kenneth Branagh, Bill Nighy, Eddie Izzard, Terence Stamp and Tom Wilkinson.

Operation Valkyrie: Operation Valkyrie (German: "Unternehmen Walküre" ) was a German World War II emergency continuity of government operations plan issued to the Territorial Reserve Army of Germany to execute and implement in case of a general breakdown in civil order of the nation.  Failure of the government to maintain control of civil affairs might have been caused by the Allied bombing of German cities, or uprising of the millions of foreign forced laborers working in German factories.

Operation Freshman: Operation Freshman was the codename given to a British airborne operation conducted in November 1942 during World War II.  It was the first British airborne operation conducted using gliders, and its target was the Vemork "Norsk Hydro" chemical plant in Telemark, Norway which produced heavy water for Nazi Germany.  By 1942, the German nuclear weapons programme had come close to being able to develop a nuclear reactor, but in order for the reactor to function it would require a great deal of heavy water.  The source of the heavy water was the Norsk Hydro plant, which had been occupied in 1940; when the British government learned of the German nuclear developments, it was decided that a raid would be launched to destroy the plant and deny the Germans the heavy water required to develop a nuclear weapon.  Several tactics were discussed and discarded as impractical, and it was finally decided that a small airborne force composed of sappers from the Royal Engineer units attached to 1st Airborne Division would land by glider a short distance from the plant, demolish it with explosives and then escape over the Norwegian border into Sweden.

Supporting paragraphs: Hans-Ulrich von Oertzen Operation Valkyrie
Answer: Claus von Stauffenberg