A one-hit KO, commonly abbreviated as OHKO (also referred to as an Instant KO in Melee), is an attack in the Super Smash Bros. games with such high base knockback that it KOs at 0%. Such attacks currently exist in every installment of the series. However, moves that have the potential OHKO are not necessarily guaranteed to do so, and may fail to OHKO under the following circumstances:
Knockback may be insufficient to KO at 0% if the attack had already been landed previously (due to Stale-Move Negation).
Knockback may be insufficient to KO heavy characters at 0%.
Opponents are under an effect that makes them more resilient (such as being giant, metal, or in an event that reduces the knockback they sustain), or the player is weakened (such as being tiny or given a lower handicap).
The damage ratio is lower than standard, reducing knockback.
The general rule for a move to be considered a one-hit KO is being able to KO an average-weight character, such as Mario, at 0% damage from the center of Final Destination under standard conditions as mentioned above. Another common standard is to compare the move's knockback to the smash attack of the Home-Run Bat, the move intended to be the quintessential OHKO by the programmers.
Moves that inflict no knockback are not considered to meet the definition of one-hit KO's; for example, being flown off the screen by the Landmaster or eaten by the Summit's Fish or Distant Planet's Bulborb are not listed below.
Brawl continued the trend of adding several OHKOs into the game. However, the majority are Final Smashes, items, stage hazards, or boss attacks, with very few being standard moves of playable characters.
Ike's Counter upon countering a ridiculously powerful attack; a hitbox that deals around 45% damage will result in an OHKO if countered. Marth's Counter can also potentially inflict an OHKO, though it must reflect an even more powerful move (a hitbox that deals around 50% damage).
Contrary to common belief, Mr. Game & Watch's Judge #9 is not an OHKO, having very high knockback but not quite enough to qualify under the required conditions.
If an uncharged Hothead is hit by a fire-based hitbox that deals over 30% damage (such as Ike's stage 7-9 Eruption), 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 as a Home Run bat swing, or being launched after receiving enough damage), it will OHKO anyone it slams into.
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, even to a character as light as Jigglypuff.
If the Hothead is powered up significantly (30% total or higher), it will gain OHKO power.
If a Soccer Ball is launched at enough speed (after either being hit by an extremely powerful attack such as a Home-Run Bat, or being launched after receiving enough damage), it will OHKO anyone it slams into.