Combat uses an Action Time Battle (ATB)-style system where characters have a recharging store of energy to use for special actions. Another way to view it is like a store of Mana for magical attacks, where the Mana gradually recharges.
In Holdara, this is called Overdrive, and represents the bio-mechs pushing themselves to do high energy actions.
...
Overdrive Actions (OA)
Based on the selected target (including no target), equipment, and skills, a context sensitive list of Overdrive Actions will be displayed along the bottom of the screen.
...
Basic attack elemental damage - inflicting elemental damage recharges OP
Energy shards - inflicting damage can dislodge energy shards that land on the ground and can be collected to recharge OP.