O
- Off-Hook
- Off-Net Call
- Office Communication Server (OCS)
- Offload
- Omni-Directional Antenna
- On-Hook
- On-Net Call
- On-Premises PBX
- One Number Service
- Online Charging System (OCS)
- Open Network
- Open Shortest Path First (OSPF)
- Open Systems Interconnection (OSI)
- Operating System (OS)
- Operational Support System (OSS)
- Operator Console
- Operator Services
- Operator-Assisted Call
- Optical Carrier (OC)
- Optical Fiber
- Optical Network Terminal (ONT)
- Optical Time-Domain Reflectometer (OTDR)
- Origination
- Outbound Call (Chiamata in Uscita)
- Outbound Routing
- Outgoing Trunk
- Outsourcing
- Over-the-Air (OTA)
- Over-the-Top (OTT)
- Overhead
- Overhead Paging
- Overlay Network
- Overload Control
- Overprovisioning
