Term | Explanation |
| Oboe | Code name for British radio-navigational aid used by RAF bombers. |
| Octagon | Code name for the second Quebec conference between Roosevelt and Churchill - September 1944. |
| Oflag | Offizier-Lager - Officer Camp German prisoner-of-war camps for Allied officers. |
| OKH | Oberkommando der Heeres - High Command of the German Army. |
| OKL | Oberkommando der Luftwaffe - High Command of the German Air Force. |
| OKM | Oberkommando der Kriegsmarine - High Command of the German Navy. |
| OKW | Oberkommando der Wehrmacht - High Command of the German Armed Forces. |
| Olive | Operational code name for the Allied assault on the Gothic Line, Italy - August 1944. |
| Omaha | Code name for one of the US Normandy beach landing areas used on D-Day, the site was between Coleville and Vierville. |
| OSS | Office of Strategic Services - US military intelligence and special operations agency. |
| Overlord | Operational code name for Allied operations in Normandy and North West Europe, beginning with Operation Neptune on D-Day, 6th June 1944. |