OC stands for optical carrier, or optical carrier rate, a standard measure of transmission bandwidth for data carried via SONET, or Synchronous Optical Networking fiber optic networks.

With a base unit of 51.84 Megabits per second (Mbps or Mbit/s), the OC transmission rate is rendered as the acronym OC followed by an integer value of the basic rate, so OC-1 indicates a transmission rate of 51.84 Mbps, OC-3 is 155.52 Mbps, and so on.

Depending on the needs of the network, different levels of OC rate can be sufficient. Smaller to mid-size businesses that need high-speed Internet service might employ anywhere from an OC-3 connection to an OC-12 connection, while Internet Service Providers (ISP), hosting services and other organizations and businesses that support a higher volume of bandwidth use could rely on one more OC-48 connections.

For the most part, classifications OC-192 and up are found as part of the Internet backbone, the principal data paths that strategically inter-link core routers and major networks on the web. SONET interfaces of up to OC-768 first became available in 2006, and an OC-768 connection was established on the TAT-14 transatlantic cable in 2008.