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, as 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 30% accuracy and a base of 5 PP. Pokémon with the ability Sturdy are unaffected by all of them, unless the user has the ability Mold Breaker. Pokémon also holding the item, Focus Sash, are not entirely affected as it leaves the user with only 1 HP, due to Focus Sash's effect.
So far, there are four OHKO moves: