==United Kingdom, Kanada dan Australia==
In the [[British Army]], [[ Royal Marines]], and many other [[ Commonwealth of Nations| Commonwealth]] military and [[ paramilitary]] organisations, the commanding officer of a unit is appointed. Thus the office of CO is an appointment.
The appointment Commanding Officer is exclusive to commanders of major units ([[regiment]]s, [[battalion]]s and similar sized units). It is customary for a commanding officer to hold the rank of [[Lieutenant Colonel]] and he or she is usually referred to within the unit simply as "the Colonel" or more commonly the CO. "The Colonel" is usually 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 Colonel]]s are CO's, and although most COs are [[Lieutenant Colonel]]s, that is not a requirement of the appointment.
units, that is, [[ company ( military unit)| company]], [[ Squadron ( army)| squadron]] and [[ artillery battery| battery]], and formations ( brigades, divisions, corps and armies) do not have a Commanding Officer.
The officer in command of a sub unit holds the appointment " Officer Commanding" or "OC". Higher formations have Commanders or General Officers Commanding (GOCs). Area commands have a Commander-in-Chief (C-in-C Land Army; C-in-C British Army of the Rhine)
The OC of a sub-unit is customarily a major, though again the rank of the appointment holder and the holder's appointment are separate and independent of each other.
In some cases, independent units smaller than a sub-unit, e.g. a platoon of Military Police that reports directly to a formation such as a brigade, will also have an OC appointed. In these cases, the officer commanding can be a captain or even a lieutenant.
Appointments such as CO and OC may have specific powers associated with them. For example, they may have statutory powers to promote soldiers or to deal with certain disciplinary offences and award certain punishments.
The CO of a unit may have the power to sentence an offender to 28 days detention, whereas the OC of a sub-unit may have the power to sentence an offender to 3 days restriction of privileges.
Units smaller than sub-units, i.e. [[platoon]]s, [[troop]]s and [[Section (military unit)|section]]s are not specific appointments and officers or NCOs who fill those positions are simply referred to as the commanders/leader. e.g. Platoon Commander, Troop Leader, Section Commander/Leader, etc.
===Tentera Udara Diraja===