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, they instead automatically fail if the target is of a higher level than the user, and their accuracy starts at 30% and increases by 1% for each level the user is above the target. For example, if the user is 70+ levels above the target, the move will have an accuracy of 100%.
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 OHKO moves any more likely to hit, and using Double Team will not make them any less likely to hit. However, any effects that cause a move to completely ignore the accuracy check (such as the Ability No Guard, or X Accuracy in Generation II or earlier) will allow OHKO moves to always hit.
A Pokémon with the Ability Sturdy will be unaffected by OHKO moves, unless that Ability is negated (such as through Mold Breaker or Gastro Acid). A Pokémon with Wonder Guard is also immune. At full HP, a Pokémon holding a Focus Sash will instead be reduced to 1 HP by any 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). However, they actually do 9999 damage, which can be reflected with IQ skills such as Counter Hitter.
List of moves