OOM

From WoWWiki

An acronym for "Out Of Mana", meaning the Player character is low or out of mana. Generally this should be a signal to other party members to pause so the character can regain some mana (sit down to have some Drink, usually). In combat it usually is a warning from a healer to the tank (or anyone taking damage) to expect limited healing for a time and that they should take health potions/healthstones/bandage if they need healing in the short-term.

The slash command /oom will do an emote conveying that you are low on mana.

Sometimes players coming from certain other MMORPGs will use the acronym OOP ("Out Of Power") instead.