Short for Optical Carrier; a prefix for SONET carrier hierarchies, which is followed by a number.

See table below for specific speeds:

OC-1: 51.84 Mbps
OC-3: 155.52 Mbps
OC-12: 622.08 Mbps
OC-24: 1.244 Gbps
OC-48: 2.488 Gbps
OC-192: 9.95328 Gbps
OC-768: 39.81312 Gbps