Operational Communications known as OPC are VHF frequencies which are dedicated for very specific uses and for which an ATCO or FISO licence or an Air/Ground Certificate of Competence is not required.
Airline company frequencies come under this as well companies operating on behalf of the MOD.
Have a look at the following.
CAP 413, Radiotelephony Manual, Chapter 9, Page 3, 1.6 Aircraft Operating Agency Messages