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 |