| O | Odd (Level with direction indicator< or >) |
| O/A | On or About |
| OAC | Oceanic Area Control |
| OACC | Oceanic Area Control Centre |
| OBST | Obstruction; Obstacle |
| OCA | Oceanic Control Area |
| OCA | Obstacle Clearance Altitude |
| OCH | Obstacle Clearance Height |
| OCL | Obstacle Clearance Limit |
| OCNL | Occasional |
| OCS | Obstacle Clearance Surface |
| OCT | October |
| OCTA | Oceanic Control Area |
| ODALS | Omni-Directional Approach Light System |
| OFIS | Operational Flight Information Service |
| OLF | Outlying Field (Navy) |
| OM | Outer Marker |
| ONC | Operational Navigation Chart |
| OPN | Open: Opening; Opened |
| OPR | Operator/operate/operative/ operating operational |
| OPS | Operations |
| O/R | On Request |
| OSV | Ocean Station Vessel |
| O/T | Other Times |
| OTR | Oceanic Transition Route (North Pacific) |
| OTS | Organised Track System |
| OTS | Out-of-Service |
| OUTBD | Outbound |
