| O & M | Operations and maintenance |
| Organisation and methods |
| O alt | Observed altitude |
| OA | Operations analysis |
| OAC | Operator access console |
| OAS | Offensive air support |
| OASIS | Onboard automatic data processing support in ships |
| Operational application of special intelligence systems |
| OAT | Operational air traffic |
| Outside air temperature |
| OB | Operating budget |
| Order of battle |
| Ore bulk carrier |
| OBAN | On-board autonomous navigator |
| OBDG | On-board degaussing |
| OBIGGS | On-board inert gas-generating system |
| OBM | Orbital ballistic missile |
| OBOC | Ore/bulk/oil carrier |
| OBOGS | On-board oxygen-generating system |
| obst | Obstruction |
| OC | Officer commanding |
| OCA | Obstacle clearance altitude |
| Oceanic control area |
| Offensive, counter-air |
| Operational control authority |
| OCH | Obstacle clearance height |
| OCIMF | Oil Companies´ International Marine Forum |
| OCL | Obstacle clearance limit |
| OCOP | Outline contingency operations plan |
| OCPB | Ocean-capable patrol boat |
| OCR | Optical character reader |
| Optical character recognition |
| OCU | Operational conversion unit |
| ODAS | Oceanographic data acquisition system |
| ODBC | Open database connectivity |
| OECD | Organisation for Economic Co-operation and Development |
| OEEC | Organisation for European Economic Co-operation |
| OEM | Original equipment manufacturer |
| Own equipment manufacture |
| OES | Outside electricity supply |
| Ofcom | Office of Communications (regulator) |
| OFIS | Operational flight information service |
| OFT | Office of Fair Trading |
| OGC | Office of Government Commerce |
| OGIS | Open geographic information system |
| OIC | Officer in charge |
| OIS | On-board information system |
| OJT | On-the-job training |
| OLAFAS | On-line automated frequency assignment system |
| OLDI | On-line data interchange |
| OLE | Object linking and embedding |
| OLED | Organic light-emitting diode |
| OM | Outer marker |
| ONC | Operational navigation chart |
| ONSOD | Omega navigation system operation detail |
| OOB | Order of battle |
| OOD | Object-oriented design |
| OOPC | Ocean Observations Panel for Climate (of UN) |
| OOTW | Operations other than war |
| OOW | Officer of the watch |
| OP | Observation post |
| OPEC | Organisation of Petroleum Exporting Countries |
| OPIS | Operational priority indicating system |
| OPR | Office of primary responsibility |
| OPRC | Oil spill preparedness, response and cooperation |
| ops | Operations |
| OPV | Offshore patrol vessel |
| OR | Operational research |
| Other ranks |
| orbat | Order of battle |
| ord | Ordnance |
| ORE | Operational readiness evaluation |
| ORI | Operational readiness inspection |
| ORTP | Operational readiness test programme |
| OS | Operating system (computer) |
| Ordinary seaman |
| Ordnance survey |
| OSB | Orientated sterling board |
| OSD | Open shelter deck |
| OSI | Open systems interchange (computer) |
| Open systems interconnect (computer) |
| OSIS | Ocean surveillance information system (satellite) |
| OSP | Online service provider |
| OST | Office of Science and Technology |
| OSV | Ocean station vessel |
| OT | Ordinary tide |
| OTAR | Overseas tariffs and regulations |
| OTEC | Ocean thermal energy conversion |
| OTHR | Over-the-horizon radar |
| OTHT | Over-the-horizon targeting |
| OTS | Operations and training support |
| OTSF | Operations and training support flight |
| OTSR | Optimum track ship routing |
| OTU | Operational training unit |
| OWS | Ocean weather ship |
| oz | Ounce (28.35 g) |
