OP
June 22 1918 Lieut Alfred Forbes Johnson to his wife Essie:
If you are not for OP [observation point] you are on night duty or taking the early morning parade at 7 o’clock, or starting an aeroplane shoot at 5 am. I seem to get about one full night a week in bed
Reading: 'The Amateur Gentleman', John Jeffery Farnol, (1916)