Proc
Project Lore Editorfrom Programmed Random Occurrence. Refers to any ability that has a chance of activating on some event such as a weapon swing or spell cast, or when that ability activates.
PK
Player Kill, when one player kills another
Pet
An NPC controlled by a player
PC
Player character, controlled by a human. Also, Personal Computer, usually referring to a computer running Microsoft Windows as the operating system.
Org
Orgrimmar, home of the Orcs
OP
Overpowered, often used as a complaint that an ability or class is imbalanced to a point that it is unfair.
OOM
Out of Mana
OMW
On My Way
NPC
Non player character. The characters are controlled by the server or realm. A "computer" character.
Ninja
Taking an item, usually loot, without permission or agreement.