Observation Balloon. A captive balloon behind the lines which observes the enemy. The enemy doesn't mind being observed, so takes no notice of it. It gives someone a job hauling it down at night, so it has one good point.
Observation Post. A position in the front line where an artillery officer observes the fire of our guns. He keeps on observing until a German shell observes him. After this there is generally a new officer and a new observation post.
O. C. Officer commanding.
Officers' Mess. Where the officers eat the mess that the O. S. have cooked.
O. S. Officers' servants. The lowest ranking private in the Army, who feeds better than the officers he waits on.
"Oil Cans." Tommy's term for a German trench mortar shell, which is an old tin filled with explosive and junk that the Boches have no further use for.
"One up. " Tommy's term for a lance-corporal who wears one stripe. The private always wonders why he was overlooked when promotions were in order.
"On the mat." When Tommy is haled before his commanding officer to explain why he has broken one of the seven million King's regulations for the government of the Army. His "explanation" never gets him anywhere unless it is on the wheel of a Umber.
"On your own." Another famous or infamous phrase which means Tommy is allowed to do as he pleases. An officer generally puts Tommy "on his own" when he gets Tommy into a dangerous position and sees no way to extricate him.
Orderly-Corporal. A non-commissioned officer who takes the names of the sick every morning and who keeps his own candle burning after he has ordered "Lights out" at night.