It's from the phonetic alphabet.
The letter C is spoken as Charlie
Oscar is simply "O" but expressed as part of the phonetic alphabet. For example, Alpha, Bravo, Charlie, Delta, etc.
This is not used in the U.S. military.
Wanna know what Charlie Daniels plays? Are you sure? Ok, well he plays A MEAN FIDDLE!!!!!
Bronetransporter
secured area
It means to be armed
To be forced into service
Died While Missing
Extended combat training
north Atlantic treaty organization
It is "Basic Active Service Date"
member of a State national Guard Unit