An option select (shortened OS) is a technique where the player performs a set sequence of inputs that will give different results depending on the situation or the action done by your opponent. The game "selects" automatically for the player the outcome and ensures therefore that the player will always end up in a safe or favorable situation. As such, OS circumvent the need of guessing what your opponent will do next.
Option selects are generally exploits of specific properties or bugs of the game engine where it has to decide for ambiguous situations. Their presence is often unavoidable without removing or limiting other positive features of the engine such as input leniency. While games like Street Fighter V are designed to limit as much as possible the number of OS in order to encourage reading, some games other like Guilty Gear Xrd actively support the use of OS by displaying them in training mode.
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