From Bulbapedia, the community-driven Pokémon encyclopedia.
One-hit knockout moves, commonly abbreviated as OHKO moves, are moves that, when used, cause the target's HP to drop to zero regardless of their stats. They are all highly inaccurate: given that their effect is to immediately make the target faint, to have them much more accurate would further encourage their use to quickly and easily finish battles.
When used, they will always fail if the level of the target is higher than the level of the user, except in Generation I, where they would fail if the user's Speed stat was lower than the target's.
All one-hit knockout moves have a base of 5 PP. Its accuracy is at least 30% and grows slightly the lower the target Pokémon's level is relative to the attacking Pokémon. A level difference of 70 will deliver an accuracy of 100%. It is not affected by changes in accuracy or evasion stats and is calculated as follows:
Accuracy = ((User's level - Target's level) + 30)%
Pokémon with the ability Wonder Guard are unaffected by them, unless the user has the ability Mold Breaker or a variant of it. A Pokémon with Sturdy is also immune. At full HP Pokémon holding a Focus Sash will instead be reduced to 1 HP by a OHKO move.
List of moves