What does CO mean in the Army?
Commanding officer
In the British Army, Royal Marines, and many other Commonwealth military and paramilitary organisations, the commanding officer of a unit is appointed. Thus the office of CO is an appointment.
The appointment of commanding officer is exclusive to commanders of major units (regiments, battalions and similar sized units). It is customary for a commanding officer to hold the rank of lieutenant colonel, and they are usually referred to within the unit simply as "the colonel" or the CO. "The colonel" may also refer to the holder of an honorary appointment of a senior officer who oversees the non-operational affairs of a regiment. However, the rank of the appointment holder and the holder's appointment are separate. That is, not all lieutenant colonels are COs, and although most COs are lieutenant colonels, that is not a requirement of the appointment.