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