Killer Move: Serious Series

Killer Move: Serious Series (マジシリーズ, Hissatsu Majishirīzu) is a fighting style used by Saitama which consists of a series of his "serious" moves.

Overview
Whenever Saitama decides to put more effort into his attacks, he makes use of these moves which are dubbed as "serious". His face usually looks serious as he performs these moves. Overall, the clear difference between Saitama's "serious" and "normal" attacks is the massive increase in power and destructive capability. Saitama does not actually consider these moves to be anything special. Like Saitama's normal techniques, this fighting style is improvised and usually made up on the spot while in battle.

While fighting Saitama, Garou, in Cosmic Fear Mode, copied the hero's Serious Punch and its immense power using Mode: Saitama, making him the second user of this unique fighting style. Though, Garou has only been shown using Serious Punch.

Known Moves

 * Serious Punch (マジり, Maji Naguri): The first serious move used by Saitama in the series. He simply throws an extremely powerful punch. One of his serious punches was so powerful that not only did it completely negate Boros's Collapsing Star Roaring Cannon, a beam capable of wiping out an entire planet, but the shockwave from the punch alone was also powerful enough to part the clouds on a global scale, as well as deliver the final fatal blow to Boros. The dying Boros also declared that Saitama's fatal attack on him was not even anywhere near Saitama's true power and that Saitama was still holding back. Saitama also used this move to completely annihilate and disintegrate Elder Centipede, a Dragon-level threat and one of the biggest monsters in the series. When used against Evil Natural Water, who has become one with the ocean, he was able to dissipate its form and split the ocean entirely. When used against Cosmic Fear Mode Garou, their Serious Punch collided and created a Serious Punch², which caused a shockwave of energy powerful enough to destroy the Earth and had to be redirected by Blast and his associates in order to spare the planet. Despite being diverted, the energy released from their punches could travel into outer space where it (presumably) obliterated a large number of star systems in its path within mere seconds, with the recoil launching both Saitama and Garou to Jupiter's moon Io.
 * Omnidirectional Serious Punch (マジり, Zenhōi Maji Naguri): Saitama leaps from multiple angles to create a series of afterimages which all then surround and strike the opponents with a Serious Punch, seemingly leaving no room for them to escape. He first used this attack against Cosmic Fear Mode Garou, who attempted to flee through a hyperspace portal to avoid the attack.
 * Serious Side Hops (マジ反復横とび, Maji Hanpukuyokotobi; Viz: Serious Sideways Jumps): Saitama rapidly hops from side to side, creating a wall of afterimages. He is able to walk forward while doing so, and the shockwaves caused by it were enough to easily render unconscious Speed-o'-Sound Sonic, even with his 10 afterimages.
 * Serious Squirt Gun (マジ, Maji Mizudeppō): Saitama uses his hands in liquid to squirt a super high-pressured stream. He used this move while submerged in lava to completely overpower Orochi's Gaia Cannon which was an extremely powerful attack that drew energy from the very core of the Earth itself.
 * Serious Table Flip (マジちゃぶし, Maji Chabudai-Gaeshi): Saitama digs his fingers into the ground and flips a large amount of ground thousands of meters into the air. He first used this technique against Garou while they were on Io, which completely ripped apart the moon. The effect was so great that Garou's sense of gravity and balance was severely warped.
 * Serious Sneeze (マジ, Maji Kushami): Saitama simply lets out a sneeze. In his fight with Garou on Io, one of Jupiter's moons, Saitama's strength increased enough that a mere sneeze from him ended up devastating the large planet Jupiter when struck.
 * Serious Headbutt (マジ頭突き, Maji Zuzuki): Saitama headbutts his target. In the webcomic, he uses this move to counter a punch from Garou and break his arm.

Trivia

 * In the original webcomic, there were only Serious Punch, Serious Side Hops, Serious Table Flip and Serious Headbutt.
 * Murata once joked that if Saitama missed the moon in his fight with Boros, Saitama could use a serious fart to get back to earth.