Off Peak
The period of time after the business day has ended during which carriers may offer reduced airtime charges.

OFTEL
The Office of Telecommunications. Independent watchdog for the UK telephone and mobile industry.

OGM
Outgoing message. What the caller hears when they dial your answer machine or voice-mail box.

One-touch memory
Button on a phone that allows you to dial a number just by pressing this one key. Also known as a fastkey.