LOM/OOM is an acronym for Low on Mana and 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 (also known as an emote) /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.