During World War II he served in the U.S. Army Signal Corps until July 2, 1945, when he was discharged.
O'Donnell bribed the Soviet soldier guarding the entrance to Hitler's Berlin bunker becoming the first non-Soviet to examine it.
After using these documents and interviews with many of the last occupants of the Führerbunker in his later publications, he became an authority on the death of Adolf Hitler, and ultimately published his collected findings in his 1975 book, The Bunker.
In 2011, historian Niall Ferguson credited O'Donnell as one of the few Western observers who accurately foresaw the fall of the Berlin Wall.
[1] In the 1981 CBS television movie The Bunker, O'Donnell is portrayed by actor James Naughton in a brief scene at the beginning.