OAW: Oesterreichische Albatros Werke, Austrian manufacturer of aircraft.
OB: Order of Battle.
Obergefreiter: German artillery bombardier.
Oberjäger: German Jäger corporal.
OC: Officer Commanding.
October Manifesto: A declaration issued by Emperor Karl I of Austria-Hungary which changed parts of the Empire into a ‘federation’ of semi-autonomous states. It was swiftly rejected.
Oeffag: Oesterreichische Flugzeugfabrik, Austrian manufacturer of aircraft.
Offizier-Aspiranten : German volunteers for a one-year reserve commission.
Offizierstellvertreter : German acting officer.
OHL: Oberste Heeresleitung, German General HQ.
Oil: ANZAC colloquialism for the truth.
Oil can: German mortar bomb.
Old and bold: Experienced soldiers.
Old eyes: British Guards.
Old pot and pan: Commanding Officer.
Old soldier: slang for someone skilled at avoiding duty.
Old sweat: An experienced soldier.
OO: Ordinance Officer.
Oofs: Eggs.
Oojah: Slang used where the right name wasn’t known.
Opanka : Bulgarian sandals.
O Pip: Observation post.
Opolchenie : Russian militia.
O/R: Other ranks (not officers).
Orderly: Medical attendant or any soldier allocated a special duty.
Original: A unit’s original members.
Orion: Class of British battleship, nicknamed the Super Dreadnoughts.
OS: Ordnance Services.
OTC: Officers’ Training Corps.
Otranto Barrage: Allied Attempt to block the Straits of Otranto using trawlers and Indicator Nets.
O-UFA: Oesterreichische-Ungarische Flugzeugfabrik Aviatik, Austrian manufacturer of aircraft.
Over the Top: Climbing up out of a trench and attacking.