Short answers, pulled from the story.
The Oberkommando des Heeres, abbreviated OKH, was the high command of the German Army under the Nazi regime. Founded in 1935 as part of Hitler's rearmament program, it was responsible for strategic planning of Armies and Army Groups during World War II. It was de facto the most important unit in German war planning until the defeat at Moscow in December 1941.
OKH was the Army High Command, focused on strategic planning for ground forces; OKW, the Oberkommando der Wehrmacht, was the Armed Forces High Command created in February 1938 to sit above OKH, the Luftwaffe, and the Navy high commands. After Moscow in December 1941, OKH's authority was limited to the Eastern Front while OKW took direct control of army units in all other theatres.
The Commander-in-Chief of the Army, the Oberbefehlshaber des Heeres, headed OKH. Field Marshal Walther von Brauchitsch held the post until the crisis at Moscow in December 1941, when he was dismissed partly because of his failing health. Hitler then appointed himself as Commander-in-Chief of the Army.
Both OKH and OKW were headquartered at the Maybach complex, a set of camouflaged facilities in Nazi Germany. OKH occupied Maybach 1 and OKW occupied Maybach 2, physically separated by a fence. Despite sharing the same compound, personnel noted that the two staffs maintained structurally different mindsets and largely independent operations.
On the 28th of April 1945, two days before Hitler's suicide, Hitler formally subordinated OKH to OKW, giving OKW command over the Eastern Front. This ended the operational separation that had kept OKH notionally distinct since the Moscow defeat in December 1941.
In 1944, OKH's principal officers included Friedrich Fromm as Commander-in-Chief of the Reserve Army, Friedrich Olbricht as Chief of the General Army Office, Emil Leeb leading the Army Ordnance Office, Rudolf Schmundt overseeing the Army Personnel Office, and Heinz Guderian as Inspector General of Armoured Troops. Karl-Wilhelm Specht served as Inspector General for Officer Cadets.