One-hit KO

A one-hit KO, commonly abbreviated as OHKO, is an attack in the  games with such high base knockback that it will KO an opponent at 0%. Such attacks currently exist in every installment of the series.

Definition
In general, a move is considered a one-hit KO if it inflicts enough knockback to KO Mario at 0% from the middle of Final Destination (Dream Land in Smash 64).

As a result, moves and situations like the following are not considered to be one-hit KOs:
 * Meteor smashes that only work offstage, such as Luigi's down taunt
 * Moves or hazards that inflict no knockback, such as being flown off the screen by the Landmaster or eaten by the Summit's Fish or Distant Planet's Bulborb
 * Sacrificial KOs such as Bowser's Flying Slam, due to also inflicting no knockback
 * Moves that only work on certain light characters (such as the first hit of 's Blazer against on specific stages in Melee)
 * Moves that only work on certain stages (such as Ike's Great Aether on 75m), with stage hazards being an exception
 * Instant KOs such as Hero's Whack, due to inflicting no knockback

Conversely, a one-hit KO move may fail to KO if the standard conditions are not met, such as the stage being large or having walls; the opponent being at the far end of a stage; the move being weakened due to stale-move negation; or the opponent using DI, momentum cancelling, crouch cancelling, or various other techniques to reduce the knockback or alter the trajectory of the attack.

In Super Smash Bros.
Moves that KO at 0% are almost nonexistent in the original Super Smash Bros. In fact, aside from the Home-Run Bat, only one move in the game is able to OHKO, and only under uncommon conditions involving items.

Items

 * A smash attack with the Home-Run Bat.
 * If uses  on a walking Bob-omb, he (as well as any character caught in the blast radius) will be OHKO'd.
 * If two Foxes use Reflector on a Green Shell, it will eventually gain OHKO power after 3-4 reflects.

Video
The following video demonstrates the main moves that KO at 0% in Smash Bros. for the Nintendo 64:

Playable characters

 * 's fully charged Flare Blade.
 * Roy's Counter (sweetspot) upon countering a hitbox that deals at least ~35% damage.
 * 's fully loaded Oil Panic after absorbing three strong projectiles (such as three PK Flashes).

While close, Mr. Game & Watch's Judge #9 is not quite strong enough to KO at 0%.

Items/Enemies

 * A smash attack with the Home-Run Bat.
 * Touching Moltres.
 * Articuno's Icy Wind.
 * Getting caught in Lugia's Aeroblast (if most hits connect). Touching Lugia's body can also lead to an OHKO if multiple hits chain (while a single hit KOs as low as 35%).
 * A thrown Polar Bear (smash throw).
 * A Green Shell gains OHKO power when reflected 3 or 4 times by two es using Reflector.

Stage hazards

 * Rapid laser shots from the Great Fox on Corneria, as well as the turrets while the Great Fox is charging.
 * On Flat Zone, if a falling tool is launched at enough speed (after either being hit by an extremely powerful attack such as a Home-Run Bat swing, or being launched after receiving enough damage), it will OHKO anyone it slams into.
 * Banzai Bill's explosion on Princess Peach's Castle (if all hits connect).
 * Touching a blue Klaptrap on Jungle Japes.

1-P Mode

 * and 's Crush (if all hitboxes land), Power Punch, Flying Punch (sweetspot), Jetstream (sweetspot), and Flying Slap ( Very Hard difficulty)
 * 's Reverse Throw, Master Hand's Finger Drill (when preparing to launch), and Crazy Hand's Fake-Out Slap (Classic Very Hard).
 * The combined hands' Applause (if all hits connect) and Sandwich Punch (Classic Very Hard).
 * In Cruel Melee, the Fighting Wire Frames' forward smash, up smash, and other moves.

Video
The following video demonstrates nearly all the moves and hazards that KO at 0% in Melee:

In Super Smash Bros. Brawl
Despite The Subspace Emissary automatically increasing the player's gravity and falling speed by up to 19% from normal brawls (making OHKOs less likely), Brawl features the largest amount of OHKOs, with the vast majority being from bosses.

Playable characters' standard moves
’s Chain with the Chain Jacket glitch can also copy another hitbox, including a hitbox of OHKO power.
 * 's stage 8 Eruption. Requires the freshness bonus and therefore does not work in Training mode.
 * 's fully loaded Oil Panic after absorbing three medium power projectiles (such as three PK Flashes) or a very strong one.
 * The following counterattacks:
 * Ike's Counter (sword and body hitboxes) upon countering a hitbox that deals at least ~45% damage.
 * 's Counter (sword and body hitboxes) upon countering a hitbox that deals at least ~50% damage.
 * Any sufficiently strong or repeatedly reflected projectile when reflected or countered, such as 's Din's Fire, or a Green Shell.

Final Smashes

 * Marth's Critical Hit.
 * 's Aura Storm at high enough Aura (Lucario's damage must be at least 95%) if all hits connect, including the initial take-off.
 * 's Triple Finish if most hits connect, including the last one.
 * 's Beast Ganon if most hitboxes land (at least one stomp and charge).
 * Zelda's Light Arrow (first opponent only).
 * ' Zero Laser if all hits connect.
 * 's End of Day if all hitboxes land (bury, ascent, and explosion).
 * 's Blue Falcon.
 * Wario-Man's Wario Waft fully charged.

While close, 's Super Jump Punch during Negative Zone is not quite strong enough, only KOing Mario at 5% and Bowser around 7%. Negative Zone inflicts incremental damage and can easily break these thresholds, but this damage does not consist of "hits" due to inflicting no knockback.

Items

 * The Dragoon.
 * A smash attack with the Home-Run Bat.
 * Touching Moltres.
 * Suicune's Aurora Beam.
 * Deoxys's Hyper Beam.
 * Electrode's Explosion when smash thrown. If an Electrode fails to burst, it can still be picked up and smash thrown with OHKO power.
 * If runs over a buried Pitfall while riding on his Wario Bike, he will be sent flying at OHKO power.
 * If an uncharged Hothead is hit by an electric or flame-based hitbox that deals at least 30% damage (such as 's stage 6-9 Eruption), it will OHKO when smash thrown or launched. Coming into contact while it is sparking will also OHKO.
 * If a Soccer Ball is launched at enough speed (after either being hit by an extremely powerful attack such as forward smash or a Home Run bat swing, or simply being launched after receiving enough damage), it will OHKO anyone it slams into.

Stage hazards

 * The Ultimate Chimera's bite on New Pork City.
 * If the the bite lands on the mouth's edge, it deals the same immense knockback of Marth's Critical Hit. If the bite traps the character inside the mouth (or the Ultimate Chimera turns around to inflict a reverse bite), both knockback and damage are even higher, rivalling Tabuu's Off Waves on the higher difficulties.
 * Corneria's laser beams.
 * Touching the hull of the Pirate Ship.
 * The first and fourth spikes on Rumble Falls (can easily be teched).
 * Touching a Klaptrap on Jungle Japes.

Bosses
Due to Boss Battles mode weakening bosses (to compensate for the player's single stock and lack of continues, items, or stickers), all of the following apply only to or The Subspace Emissary, and cannot OHKO in Boss Battles mode on any difficulty with a single exception. Tabuu's Off Waves will still OHKO in Boss Battles on Intense difficulty.
 * and 's Reverse Throw (Intense).
 * Rayquaza's Iron Tail (low HP, sweetspot/tip, Intense). Can be teched.
 * Galleom's Collapse (sweetspot, Very Hard).
 * Galleom's Collapse (sourspot), Double Lariat (all hits connect), Giant Jump (landing directly on opponent, low HP), Uppercut (sweetspot, low HP), and Rush & Slam (against grounded opponents, low HP) (all Intense).
 * Meta Ridley's Mega Fire Ball (low HP) and Falcon Flyer Pull (giant fireball at point-blank) (all Intense).
 * Tabuu's Off Waves (any difficulty at less than half HP, Normal at any HP).
 * Tabuu's Movement Throw (Hard), Tabuu Shot (Very Hard), Blade Charge (less than half HP, Very Hard), Shadow Laser (Intense), Discharge Attack (body, Intense), Tabuu Fire (Intense).

Other

 * In Cruel Brawl, the Red Alloy's forward smash can OHKO from the center of the stage even on.

Video
The following video demonstrates some moves and hazards that KO at 0% in Brawl:

This video shows some other OHKOs, including more boss attacks.

Playable characters' standard moves

 * 's fully charged and almost fully charged Flare Blade (sweetspot).
 * 's fully loaded Oil Panic after absorbing three medium power projectiles or a very strong one.
 * 's Pocket after it has absorbed a particularly strong projectile, most notably Timber. Powerful projectiles released by Pocket that can still be reflected will also OHKO, such as 's fully charged Phantom Slash.
 * Many other reflectors also share this behaviour, especially 's bat.
 * The following counterattacks:
 * Ike's Smash Counter and 's Power Vision custom moves upon countering a hitbox that deals 30% damage or more.
 * 's Counter Surge, Ike and Roy's Counter, and Shulk's Vision upon countering a hitbox that deals 35% damage or more. These counters technically have varying degrees of knockback, but they nevertheless achieve a KO at 0% without DI.
 * 's Blade Counter upon countering a hitbox that deals 37% damage or more.
 * and 's Counter, 's Slip Counter, and 's upon dealing the 50% damage cap with their counters (this requires countering a hitbox that deals at least ~38.5% damage for Little Mac and Palutena, and at least 40% damage with Lucina and Marth).

Contrary to its name and intention, Little Mac's KO Uppercut does not KO until above about 30% damage. At 0%, it falls short of the necessary amount of knockback, KOing a character as light as at 12%.

With Rage applied

 * 's fully charged forward and up smash (due to aura and rage stacking).
 * Ganondorf's reverse and/or aerial Warlock Punch.
 * 's Giant Hammer.

Final Smashes

 * Marth and Lucina's Critical Hit.
 * Some of Giga Mac's attacks when fully charged and sweetspotted:
 * Straight Lunge, up smash and forward smash (when angled up or horizontally).
 * Shulk's Chain Attack when boosted with the Hyper Smash Monado Art (extremely frame-precise, as the final hit must land before the Art expires).
 * 's Blue Falcon (barely in OHKO threshold, easily DIed by holding towards the stage).
 * Wario-Man's fully charged Wario Waft.

Items

 * The Dragoon.
 * A smash attack with the Home-Run Bat.
 * Electrode's Explosion after being smash thrown.
 * If a Hothead is powered up significantly by a flame- or electric-based hitbox (30% total or higher), it will gain OHKO power when smash thrown or emitting sparks.
 * Equipment can increase the player's damage, increasing the amount of damage a Hothead can be charged by and thus making it easier to reach OHKO power.
 * The Black Knight item from Smash Tour will cause smash attacks to have a 30% chance to have 5 times knockback, occasionally turning them into OHKOs.

Stage hazards

 * Touching the hull of the boat on Wuhu Island.
 * Touching the hull of the Pirate Ship.
 * Touching a Klaptrap on Jungle Japes.

Bosses

 * Master Core's final wave attack (on 8.0 or higher intensity only). Ironically, the chances to survive are higher if the player is closer to the blast lines.
 * Master Core's self-destruction after unleashing the last wave (very small hitbox). However, this knockback can never defeat the player as they have already won by the time the hitbox lands.

Other
While it isn't accessible in regular battles, the bomb used in the Target Blast stage has enough power to OHKO.

Video
The following video demonstrates all the moves and attacks that KO at 0% in Smash 4:

Playable characters' moves

 * and 's fully charged and almost fully charged Flare Blade (sweetspot).
 * 's fully charged forward smash with Oomph or Psyche Up active (sweetspotted critical hit).
 * 's reversed aerial Warlock Punch (after inhaling ).
 * The following counterattacks:
 * Roy and Chrom's Counter, 's Tetrakarn with Arsène, 's Vision with Smash Art, and 's Counter Throw countering a hitbox that deals around 35% damage or more.
 * 's Counter Surge and 's Counter countering a hitbox dealing around 40% damage or more.
 * and 's Counter countering a hitbox dealing around 43% damage or more.
 * Any sufficiently strong or repeatedly reflected projectile when reflected.
 * Any throwable item under extreme conditions which cause them to move at a very high speed. An example is 's fully charged Gyro.

With Rage applied

 * Hero's Hatchet Man with Psyche Up active.
 * Hero's Kazap with Psyche Up, Oomph, and Hocus Pocus Giant active.
 * 's Oil Panic after absorbing a very strong projectile (like Hero's Kafrizz).
 * Mr. Game & Watch's Judge #9.
 * Ganondorf's reversed aerial Warlock Punch.
 * 's fully charged forward smash at max aura after patch 9.0.

Final Smashes

 * Marth and Lucina's Critical Hit.
 * Lucario's Aura Storm at max Aura if all hits connect.
 * 's End of Day if it hits a fighter that cannot be buried. End of Day's bury hitbox deals 633.15 units of knockback, more than enough to KO any character on Ultimate's Training Mode Stage.

Final Smashes executed using Final Smash Meter always lose their OHKO power, with Critical Hit KOing at 20%.

Items

 * The Dragoon.
 * A smash attack with the Home-Run Bat.
 * Electrode's Explosion when smash thrown.
 * A Hothead when brought from no charge to significant overcharge by a single flame or electric attack that deals over 50% damage. If the Hothead hits while sparking, it can OHKO from attacks closer to 30%.
 * Spirits can increase the player's damage, increasing the amount of damage a Hothead can be charged by and thus making it easier to reach OHKO power.
 * The Deku Nut's explosion on a grounded target that cannot be stunned, such as Sandbag, or Kirby in his Stone transformation.

Stage hazards

 * Touching the hull of the Pirate Ship.
 * Touching the hull of the boat on Wuhu Island.
 * Being hit by a Klaptrap on Jungle Japes.

Other
While inaccessible in a regular fashion without hacking, the gun found on the Free the Spirit screen's stage is programmed in as its own usable item, inflicting 30% damage with OHKO power (mainly because the Free the Spirit stage is very small with close blast zones). This situation is similar to Smash 4's Target Blast bomb.

Video
The following video demonstrates all the moves and hazards in Ultimate that inflict enough knockback to KO at 0%, alongside some powerful moves that don't. However, it also includes a few moves and hazards that do not inflict any knockback, such as Instant KOs:

Conclusive video
The following video demonstrates several one-hit KOs from every Smash game except Ultimate:

Trivia

 * In Melee, 's trophy refers to Flare Blade as an "instant KO".
 * The Home-Run Bat is the only item (and by extension, the only attack) able to perform a guaranteed OHKO in every Smash game, at least on its.
 * Roy's Flare Blade and Counter are the only moves to maintain their OHKO power in every installment.
 * At least one playable character from the universe possesses a move with OHKO potential in every game. Other main characters appear as items with OHKO power (Black Knight in Smash 4).
 * Marth's Critical Hit is the only Final Smash to maintain its OHKO power in every installment. Throughout all games, it is also the most powerful attack of any character's standard moveset (in terms of knockback) without any exterior factors or buffs, being impossible to survive if in direct sight of a blast line, even on the largest stages of each game.
 * While the majority of OHKOs will fail to work on characters under the effects of a Super Mushroom or Metal Box, a few attacks have such high knockback that they will still OHKO under these circumstances. In Brawl, this includes Suicune's Aurora Beam, Marth's Critical Hit, the Ultimate Chimera's bite, and several boss attacks.
 * Starting from Smash 4, a few Final Smashes have hitboxes that negate the effects of such buffs, such as Marth's Critical Hit.
 * Tabuu on Intense difficulty in The Subspace Emissary has more OHKOs than any other boss or enemy in any Smash game, with 7.
 * The most powerful moves in the Smash series are Tabuu's Off Waves (waves and contact with his body) on Intense difficulty in The Subspace Emissary and Boss Battles, and the Ultimate Chimera's bite in Brawl. Both these attacks are the most damaging moves, dealing 100%-200% damage and having immense knockback.
 * Master Core's final explosion on 8.0 difficulty or above is the only OHKO that will never KO the player, since the battle is always already won by the time the hitboxes land.
 * Galleom's Tank Rush and Tank Rush & Slam can also hit the player after the boss is defeated, with OHKO power on the higher difficulties.