A one-hit KO, commonly abbreviated as OHKO (also referred to as an Instant KO in Melee), is an attack in all three Super Smash Bros. with such high base knockback, that it KOs at 0%. However, almost all of the so-called OHKOs are highly conditional; they all share the following restrictions:
- The opponent does not tech, DI, momentum cancel, and/or crouch cancel.
- There is no wall to absorb the knockback.
- Knockback may be insufficient to KO at 0% on very large stages (e.g. Hyrule Temple, New Pork City, 75 m.).
- Knockback may be insufficient to KO at 0% if the attack had been landed previously (due to Stale-Move Negation).
- Knockback may be insufficient to KO heavy characters at 0%.
- The opponent is not giant and/or metal.
- The damage ratio is on 1.0 and no handicaps.
The typical rule of one-hit KO's is being able to KO the majority of characters from the center of Final Destination at 0%.
- A smash attack with the Home-Run Bat.
- Roy's almost fully or fully charged Flare Blade.
- Mr. Game & Watch's fully loaded Oil Panic after absorbing three strong enough projectiles (Such as three PK Flashes).
- Ike's stage 8 Eruption.
- Roy, Ike, or Marth (Brawl only) countering a damaging enough hitbox (much more likely to be an OHKO with Roy).
- Marth's Critical Hit.
- Beast Ganon (If all hitboxes land).
- Puff Up, depending on stage size (more likely to be OHKO on smaller stage if the opponent is close to its body).
- Wario-Man's fully charged Wario Waft.
- Lucario's Aura Storm when it's at full power (170% damage).
- Zelda's Light Arrow (barely in OHKO threshold, easily DIed).
- Getting hit by a walking Bob-omb after being reflected by Fox's reflector in SSB.
- Lugia's Aeroblast (only in Melee).
- Touching Moltres.
- Suicune's Aurora Beam.
- Deoxys's Hyper Beam.
- When smash thrown, Electrode's explosion in Brawl is strong enough to OHKO nearly every character.
- The Dragoon.
- If Wario is striking a buried Pitfall while riding on his Wario Bike, he will be sent flying for an OHKO.
- If a Hothead is hit by a fire-based attack that does over 30% (such as Ike's stage 7-9 Eruption) while it is uncharged, it will OHKO when smash thrown. Any sparks it gives off while traversing the stage at this strength will also OHKO.
- If a Soccer Ball is launched at enough speed (after either being hit by an extremely powerful attack such a Home Run bat swing, or being hit after receiving enough damage), it will OHKO anyone it slams into.
- Banzai Bill's explosion on Princess Peach's Castle (if all hits connect).
- Great Fox's cannon laser shots on Corneria.
- Touching the hull of the Pirate Ship.
- The Ultimate Chimera's bite.
- The first spike on Rumble Falls (can easily be teched).
- Getting crushed in the Subspace Emissary.
- Tabuu's Off Waves (on Normal or higher difficulties in SSE) (Very Hard and Intense in Boss Battle Mode).
- Tabuu's Golden Bracket attack on Intense difficulty in the SSE.
- Galleom's attack when he stiffens and falls on the player on Intense difficulty in the SSE.
- Tabuu's Shark Blade attack on Intense difficulty
- Master Hand's grab (up throw), third poke, jet, and drill (the start up of his attack) (his palm, and his wrist can also OHKO, when he drills to the ground). Crazy Hand's drill (same as Master Hand's), rocket punch, jet (only when he attempts to fly into the player), third poke, grab (forward throw), and punch (capable of KOing grounded player). All of their combination attacks can KO at 0% (only in classic mode on very hard difficulty in Melee).
- The smash attack with the Home-Run Bat is the only attack to be an OHKO in all three installments of Super Smash Bros..