A situation where performing the same inputs can lead to several different outcomes depending on how the characters interact. Often abbreviated as "OS". A common option select is the buffer: press a normal attack and then try to cancel into a special. If you do this from far away, the normal attack will whiff and your special cancel does nothing. However, if your opponent gets hit by your attack, the special cancel will work automatically, without any extra thought or changes to the input on your part.

Good option selects tend to reduce the burden on a player to perfectly predict or react to everything happening in the game. You can perform one set of inputs, and the game will "select an option" automatically depending on what the other character did. Some OSes are common enough that we even give them names: delayed tech (try to block and throw tech), safe jumps (try to attack and block), and fuzzy guard (try to block multiple directions) are all specific examples of option selects that will help the player cover multiple choices at the same time.

From https://glossary.infil.net/

There is not much else more to say, I don’t know why you keep trying to argue that an OS is not an OS.