The Overdrive Meter is divided into 3 tiers. The Lv. 1 Over Skills are generally the weaker ones, but have the most immediate accessibility. The skills grow progressively stronger the higher tier they are. Also note that the skill's strength is dependant on the item used and the stats of the character.
The meter is filled as damage is either inflicted or received; multi-hitters increase it much faster. The meter also depletes when an OS (Over Skill) has been used; the amount is relative to the tier of said skill. For example: if the Overdrive Meter is fully charged, but a Lv. 2 OS is used, it depletes down to the first tier, allowing another character to follow up with a Lv. 1 skill if possible. If a Lv. 3 skill is used instead, however, it is dropped down to zero. Also note that the individual hits in a skill can still add to the Meter.
The use of an Execution Level skill will destroy the Meter and render it useless for the remainder of the battle. It is usable once more after the battle has ended. Ex Lv skills can be used no matter what, even if the meter is at zero. There are only two of these skills: Ein's Disaresta and Ledah's Lost Seraph.
In Yggdra UnionEdit
The Overdrive Meter underwent a complete overhaul in the clashes of Yggdra Union, and was first usable in Battle Field 02. Pressing left on the control pad puts the unit into a Passive state, weakening their assault but filling the Meter. Pressing right shifts the unit into Aggression, draining the Meter as a sacrifice for the offensive boost. The Meter fills by a certain amount automatically between the clashes in a Union. The cards also effect how filled the Meter is by default when a Union begins.
The tier system has been removed entirely and the Meter must now be filled completely to use a Skill. Pressing B (GBA) or X (PSP) will activate a charging sequence which can be interrupted by a heavy attack from the opposing party; how long it takes depends on the Skill itself. The charging can be skipped, allowing for the Skill's immediate use if the unit's equipped item permits this. The Meter will drain all the way to 0% after a skill has been used.
In both YU and Blaze Union, Genocide will shatter it in a similar fashion to Riviera's Execution Level Skills, rendering it unusable. This will prevent the use of Passive and Aggressive modes, as well as skills.