AO: Area of Operation
Dink: a term referring to a person from South Vietnam
Hump: To hike; to carry something; to carry ammunition
M & Ms: From research on other pages, I had been led to believe that "M & Ms" was a sarcastic term for medical supplies, but I now believe that I am wrong. I am not sure of the entire term, but I believe that one of the Ms refers to morphine which would be consistent with O'Brien's tone and context when his narrator suggests it is for more serious wounds.
PFC: Private First Class
PRC-25: Pronounced "prick 25"
Psy Ops: Psychological Warfare (Ops = operations)
R&R: Rest and Relaxation
RTO: Radio and Telephone Operator
SOP: Standard Operating Procedure
US KIA: United States Killed in Action
USO: United Service Organization (Volunteer Entertainment and Morale)
VC: Vietcong; a North Vietnamese Soldier