| OB | Outside Broadcasting |
| OECD | Organization for Economic Co-operation and Development |
| OFCOM | Federal Office of Communications (Switzerland) |
| OFCOM | Office of Communications (GB) |
| OFDM | Orthogonal Frequency Division Multiplex |
| OFR | Off Frequency Rejection |
| OIR | Organisation internationale de radiodiffusion |
| OQPSK | Offset QPSK |
| OR | Off-Route |
| ORB | Orbit (used in the designation of certain WARCs) |
| OSCAR | Orbiting Satellite Carrying Amateur Radio |
| OTH | Over-the-Horizon |