Octet

A grouping of 8 data bits. Sometimes referred to as a Byte.

OFCOM

Office of Communications
 - The independent regulator and competition authority for the UK communications industries, with responsibilities across television, radio, telecommunications and wireless communications services.

Open Source Software

Software for which the underlying programming is available to the users so that they may read it, make changes to it, and build new versions of the software to incorporate those changes. There are many types of Open Source Software, mainly differing in the licensing term under which (altered) copies of the source code may (or must) be redistributed.

OPEX

Operating Expenditures

OSI

Open System Interconnection

Overflow Group

A feature often used to provide an overflow if the switchboard operator is busy or absent. Incoming calls are sent to the operator but other extensions have delayed ringing, so they will start ringing if the call is not answered after a pre-set time.