The Power Meter (K.O.ゲージ, K.O. Gauge) is a fighter ability unique to Little Mac in Super Smash Bros. 4 and Super Smash Bros. Ultimate that is shown above his damage meter. It is filled as Little Mac takes or deals damage (even to shields), and each level of charge (represented by the color of the bar) plays a soft, specific sound to notify the player of its charge. Once the bar is full, a loud bell sound reminiscent of those in a boxing ring plays, and the bar becomes a flashing "K.O." sign, causing his neutral special move, the Straight Lunge, to be replaced with a powerful move named the KO Uppercut. While intended to be a one-hit KO per its name, its base knockback is not quite high enough to OHKO at 0%, and reaches KO potential at about 30% when performed on the ground, although this is dependent on the weight of the character; the lightest character, Jigglypuff, can start getting KO'd at 12% and Pichu can get KO'd at 10%, where as someone like Bowser or King K. Rool do so at about 40%.
The meter is emptied when the KO Uppercut move is used, successfully or not, and immediately empties once the player presses the special button, meaning it can be depleted if the player spams the special button during hitstun. The meter will also be emptied if Little Mac is KO'd, takes a hit that causes him to tumble (except for the first four seconds after reaching full meter), or traverses through a door in Smash Run.
When Little Mac is inhaled by Kirby, Kirby copies Straight Lunge, but does not gain a Power Meter and so does not get the KO Uppercut move.
When Little Mac transforms into Giga Mac, the power meter disappears for the entire duration of the Final Smash.
The Power Meter appears in the Punch-Out!! series, where successfully landing punches to Little Mac's opponents fills the meter up, while getting hit decreases the meter. Once the meter was full, Little Mac can perform a powerful knockout blow to his opponent. In Mike Tyson's Punch-Out!! and the Wii game, the meter was replaced with the Star Punch System.