One Turn Kill (commonly abbreviated OTK) refers to acheiving a Win condition within a single turn.
A One Turn Kill occurs when the player is able to win the game within one single turn, either by dealing enough damage to reduce their life to 0, dropping their entire deck, or resolving an effect that triggers an automatic win. When performing an OTK through damage, the turn where the player wins must also be the turn where the first instance of damage occured. The most common way to perform an OTK is by setting up a field that can attack enough times with enough criticals to get through the opponent's defenses and reduce their entire life to 0. As the game of Buddyfight evolves and decks have become more powerful, OTKs have also become more common due to the very high power level of cards. On the other hand defensive plays and life gaining have also become much more efficient so many decks can also stop OTKs more reliably.
There are 3 variations of OTKs
The most common one (and at the same time, the other variations will always also be this case). In this variation the player simply manages to reduce the opponent's life to 0 in the same turn, and it doesn't necessarily have to be the player's first turn. In theory, a player could stall for over 10 turns without dealing any damage despite having chances to do so and save the resources needed to perform an OTK, if they succeed, it is still considered a legitmate OTK.
First Turn Kill (FTK)
A First Turn Kill happens when the player wins in their first turn of the game, this is much harder than a standard OTK if the player goes first since they only have their opening hand to work with and can only attack once.
Tetsuya Kurodake nearly achieved an FTK in Episode 25 on the first turn of the fight but failed. Gao's OTK can also be considered a FTK since it happened on his first turn, even though it technically was the second turn of the fight.
One Hit Kill (OHK)
One Hit Kill is the hardest variation of OTKs, as it requires the player to reduce the opponent's whole life with a single hit (i.e. a 10+ critical attack or using an automatic victory card before dealing any damage). An additional difficulty with this variation is that if the opponent somehow pays any life before the 10+ damage (i.e. calling Street Racer, Eligos) is dealt, its already not a OHK.
Rouga Aragami achieved a OHK against Tetsuya in Episode 25.