A One Turn Kill is achieved when a player wins a duel in the space of a single turn, as soon as said player has the appropriate cards in hand and has control over the duel. This is similar to a First Turn Kill, but the latter is pulled off during the player's first turn, while an OTK can be accomplished at any point of the duel (meaning that FTKs are all OTKs, but not all OTKs are FTKs).
The first prerequisite of an OTK is that it must be capable of winning in one turn, meaning you can either deplete all of your opponent's Life Points (it's normally assumed to be 8000 exactly) or fulfill a victory condition in one turn.
The second prerequisite of an OTK is that it must consist of maximum 6 or fewer cards; the listed cards cannot include cards on your opponent's side of the field and cannot include cards that relate to indefinite chance (Such as "Dangerous Machine Type-6"). All of the cards of the OTK must have a reasonable chance of being drawn early, to be able to take control of the duel and win before being overwhelmed.
The third prerequisite of an OTK is Format. Both Traditional and Advanced Formats will have very different OTK styles, owing to the allowable Card Pools.