| O |
| O&M | Operations and Maintenance |
| O&MA | Operation and Maintenance Appropriation |
| O&MA | Operations and Maintenance, Army |
| O&MF | Operations and Maintenance, Air Force |
| O&MN | Operations and Maintenance, Navy |
| O&T | Organization and Training |
| O&T(OT) | Organization and Training Directorate |
| O/H | On Hand |
| OA | Objective Area |
| OASA | Office of the Assistant Secretary of the Army |
| OASD | Office of the Assistant Secretary of Defense |
| OASD(ISA) | Office of the Assistant Secretary of Defense for International Security Affairs |
| OASD(PA) | Office of the Asst SECDEF, Public Affairs |
| OASD(SA) | Office of the Assistant Secretary of Defense (Systems Analysis) |
| OASD/ISA | Office of the Assistant Secretary of Defense for International Security Affairs |
| OASD/PA | Office of the Assistant Secretary of Defense, Public Affairs |
| OASD/SA | Office of the Assistant Secretary of Defense, Science Advisor |
| OB | Order of Battle |
| OB/GYN | Obstetrics/Gynecology |
| OBN | Project Letter Designation |
| OCB | Operations Coordinating Board |
| OCE | Office of Civil Engineer |
| OCE | Office of the Chief of Engineers |
| OCO | Office of Civil Operations |
| OCO | Office of the Chief of Ordnance |
| OCO/PSD | Office of Civil Operations/Psychological Directorate |
| OCR | Optical Character Reader |
| OCRD | Office of the Chief Rural (formerly Revolutionary) Development |
| OCS | Officer Candidate School |
| OFC | Office |
| OFF | Officer |
| OH | On Hand |
| OIC | Officer-in-Charge |
| OICC | Officer-in-Charge of Construction (USN) |
| OIR | Other Intelligence Requirements |
| OJT | On the Job Training |
| OM | Organizational Maintenance |
| OMA | Operation and Maintenance Appropriation |
| OMA | Operations and Maintenance, Army |
| OMAF | Operations and Maintenance, Air Force |
| OMN | Operations and Maintenance, Navy |
| OOMA | Ogden Air Materiel Area |
| OP | Observation Post |
| OP&A | Operations Plans and Analysis |
| OPCON | Operational Control/Condition |
| OPF | Official Personnel Folder(s) |
| OPLAN | Operation(s) Plan |
| OPM | Office of the Prime Minister |
| OPMG | Office of Provost Marshal General |
| Opn/OPN | Operation |
| OPORD | Operations Order |
| OPR | Office Pricing Responsibility |
| OPREP | Message report in joint operational reporting system |
| OPSEC | Operations Security |
| OPTAR | Operating Target(s) |
| OR | Operational Ready/Operational Readiness |
| OR/TNG | Operational Ready/Training |
| Ord/ORD | Ordnance |
| ORE | Operational Readiness Exercise |
| ORI | Operational Readiness Inspection |
| ORPS | Overseas and Return Placement Start |
| OSA | Office Secretary of the Army |
| OSA | Office of the Special Assistant (United States Mission - CIA) |
| OSA | Office of the Special Assistant to the Ambassador (US Embassy) |
| OSA | Operations Systems Analysis Office |
| OSA (PA) | Office of the Secretary of the Army (Public Affairs) |
| OSD | Office of the Secretary of Defense |
| OSD (DIO) | Office of the Secretary of Defense (Director of Information Operations) |
| OSD (I&L) | Office of the Secretary of Defense (Installation and Logistics) |
| OSD (ISA) | Office of the Secretary of Defense (International Security Affairs) |
| OSD (PA) | Office of the Secretary of Defense (Public Affairs) |
| OSD/ARPARDFUV | Projects Agency, Research and Development Field Unit, Vietnam |
| OSI | Office of Special Investigations (USAF) |
| OSP | Off-shore Procurement |
| OSS | Overseas switchboards |
| OST | Order and Shipping Time |
| OTC | Officer in Tactical Command |
| OTIG | Office of the Inspector General |
| OTPMG | Office of the Provost Marshal General |