@ Mimi Chan - For every monster you hit (given that you've selected "OVD" for the weapon/skill you're hitting them with under the skills tab in the database) your Overdrive meter will increase - you can set how much it increases by in the "MOG - XAS_Overdrive" script about 30 or 40 lines down.
When a player wants to use their overdrive (when the meter reaches 100) they simply press "Alt" and the move is executed.
When a player wants to use their overdrive (when the meter reaches 100) they simply press "Alt" and the move is executed.