CO: The abbreviated term for Commanding Officer, employed colloquially perhaps more often than "OC" (Officer Commanding). An officer of whatever rank, though referred to familiarly as the "CO" would sign officially "OC" whatever his command might be. 
References / notes
- Edward Fraser and John Gibbons (1925). Soldier and Sailor Words and Phrases. Routledge, London p.42.
Glossary of words and phrases
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