The Oberkommando der Wehrmacht was the office of the High Command of the German Armed Forces under the Third Reich from 1935 - 1945. The Armed Forces was always under the Führer, who also held the title of Oberster Befehlshaber der Wehrmacht, or literally Highest Commander of the Armed Forces. This of course was Hitler, up until his death in April of 1945 at which point Großadmiral Dönitz assumed this position. The Oberkommando der Wehrmacht was often abbreviated as OKW.
In the actual military part of the High Command, Generalfeldmarschall Wilhelm Keitel was "Chef" or Chief OKW from 1939 until the end. He was therefore the highest ranking officer in all the Wehrmacht's branchs. He supervised and coordinated the High Commands of the different divisions of the Wehrmacht: Heer, Kriegsmarine, and Luftwaffe.
The OKW and Keitel had a flag that represented the office and authority. It was a red swallow-tailed flag, with a black and white border. In the middle, inside a white circle, there were two crossed Field Marshall batons behind a German State Eagle.