A CO power is an ability which most COs in the Advance Wars series have. They can have a variety of effects, from causing massive damage, spawning units, or repairing. As such they can often change the tide of the battle. With each Advance Wars series, slight modifications have been made to CO powers, though the principal remains the same in all. Each time a player attacks with a Unit, or a Unit is attacked, they gain a small amount of Star power, which fills a bar. Once the bar is full then the players may use their COs' CO power.
Normal CO powers
The most basic CO powers and the only kind which feature in every game. As mentioned above, these vary from game to game in effects.
Super CO powers
Featuring in Advance Wars 2: Black Hole Rising and Advance Wars: Dual Strike, the Super CO Power is usually a stronger version of the original CO Power or another power entirely. Old characters from the orignal Advance Wars have kept their CO powers as well as gaining a Super CO power. Others such as Sturm and Eagle had their original CO powers made into their Super CO power, with the addition of a weaker CO power or none at all.
Tag Power/Dual Strike
Introduced in Advance Wars: Dual Strike, the Tag Power (or Dual Strike) is a special option that is available during a double CO battle when two allied CO's have their star meters full, and is able to dramatically turn the tide of a battle. When activated, it empowers all units with the current CO's Super CO power, as well as giving another turn by switching to the next CO (similar to Eagle's CO power, Lightning Strike) and activating his/her Super CO power. Certain CO pairs also affect the damage allied units will inflict, with compatible pairs increasing attack power, and imcompatible pairs reducing it.
Days of Ruin
The CO powers in Advance Wars: Days of Ruin have been revamped, and are no longer as powerful as they were in Advance Wars: Dual Strike (with some COs having no powers). Also, they can only be charged by attacking from within a certain range of a CO unit (the CO zone) or with the CO unit. Note that during an enemy phase, for the player's own meter to charge up via counterattacking, the enemy's own unit has to be in the CO Zone, but it doesn't matter if the player's unit isn't.
According to the developers, this change was made to put more emphasis on strategy, rather than on relying solely on a CO's abilities.