| OAC | Oceanic area control centre |
| OAS | Obstacle assessment surface |
| OAT | Operational air traffic |
| OBS | Observe or observed or observation |
| OBSC | Obscure or obscured or obscuring |
| OBST | Obstacle |
| OCA | Obstacle clearance altitude |
| OCA | Oceanic control area |
| OCH | Obstacle clearance height |
| OCL | Obstacle clearance limit |
| OCNL | Occasional or occasionally |
| OCS | Obstacle clearance surface |
| OCT | October |
| OFZ | Obstacle free zone |
| OHD | Overhead |
| OLDI | On−line data interchange |
| OM | Outer marker |
| OPA | Opaque, white type of ice formation |
| OPC | The control indicated is operational control |
| OPMET | Operational meteorological (information) |
| OPN | Open or opening or opened |
| OPR | Operator or operate or operative or operating or operational |
| OPS | Operations |
| O/R | On request |
| OTLK | Outlook (used in SIGMET messages for volcanic ash and tropical cyclones) |
| OTP | On top |
| OTS | Organized track system |
| OUBD | Outbound |
| OVC | Overcast |
© 2012-21 Kristina Novotna