From Bulbapedia, the community-driven Pokémon encyclopedia.
One-hit knockout moves, commonly abbreviated as one-hit KO moves or 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. In Generation I OHKO moves have a constant accuracy of 30%; however, the accuracy varies in subsequent generations, being at least 30% and increasing by 1% for every level the target is lower than the user. Therefore, 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 Sturdy are unaffected by them, unless the user has the Ability 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.
Any OHKO move will cause a Pokémon to instantly faint through 'calamitous damage'*, regardless of level, but they are prone to missing.
List of moves