From Bulbapedia, the community-driven Pokémon encyclopedia.
One-hit knockout moves, commonly abbreviated as one-hit KO moves or OHKO moves, are moves whose effect is to immediately drop the target's HP to zero. When such a move successfully hits, the game will display the message "It's a one-hit KO!" (shortened to "One-hit KO!" in Generation I).
All OHKO moves have a base PP of 5. In Generation I, their accuracy was a constant 30%, but they would automatically fail if the user's Speed stat was lower than the target's. In later generations, this was changed to instead make them automatically fail if the target was of a higher level than the user, and the accuracy would instead start at 30% and increase by 1% for each level the user was above the target. Therefore, if the user was 70 levels higher than the target, the move would always hit.
The accuracy of OHKO moves is fixed to the above formula, and will not be affected by Accuracy or Evasion stat stages. For example, holding a Wide Lens will not make a OHKO move any more likely to hit, and using Double Team will not make it any less likely to hit. However, any effects that cause a move to completely ignore the accuracy check (such as No Guard or X Accuracy in Generation II or earlier) will allow a OHKO move to always hit.
Pokémon with the Ability Sturdy are unaffected by OHKO moves, unless the user negates that Ability through Mold Breaker or a variant of it. A Pokémon with Wonder Guard is also immune. At full HP, Pokémon holding a Focus Sash will instead be reduced to 1 HP by a OHKO move.
In the Mystery Dungeon games, OHKO moves can hit regardless of level, although they are very prone to missing as always. When one hits, the game states that the target fainted through "calamitous damage" (changed to the standard "It's a one-hit KO!" in Gates to Infinity).
List of moves