What is an Option Select?
Option Select is a term for doing one or a group of commands that will do different actions depending on what the opponent chooses to do. These can take on many forms, can be simple or complex to execute, can be universal or character specific, etc.
Examples
Throw Option Select
- XRD and +R Sol inputs
- If the opponent is throwable
- Then throw takes priority and the opponent is thrown
- If the opponent is not throwable
- Then takes priority and since Sol does not have a ,
comes out instead.
A simple example is the throw option select in Guilty Gear XRD. A throw in XRD is done by pressing . Button priority in XRD goes (Throw and 
> 
> > 
> ), which means doing a command like means your character will do Throw if the opponent is in range (and not throw invincible) or if the opponent can not be thrown. The game is selecting which option to do for you depending on the situation!
Sol players often use this because Sol will do 5K (since he has no 6K command) instead of throw, which is a very fast move that can also hit opponents attempting to jump out! For characters that do have a slow , then can instead use different buttons instead, for example , which will give you close instead if they aren't throwable.
Hitstop Option Select
- BBCF Ragna inputs
~
- If opponent performs an attack at or before the expected time
- The jab will not come out as you are unable to attack while in blockstun
- If the opponent acts after the expected time
- The jab will come out
This simple OS is used to mash out of situations where it is unclear if an opponent will go into a faster move or will attempt something slower like an overhead or some unusual setup. 
should be replaced with another move if the character in question has a faster normal that will hit crouching opponents.
Reverse DP Input
- BBCF Ragna inputs
- If the opponent crosses you up
- Your input gets reversed and
(the input for Inferno Divider) takes priority - If the opponent does not cross you up
- You input gets interpreted as a Barrier Input
A slightly more complex example is a reverse dp input to deal with crossups: Ragna's invincible reversal is Inferno Divider. However, this input is reversed when the opponent crosses you up. If you suspect the opponent may cross you up and you want to Inferno Divider if he does, you can do the input . This is useful since you will be able to either hit the opponent if they cross you up or block if they do not. Again, you do different actions depending on what the opponent does!
Also while the example ends with you crouch blocking, you can easily switch to stand barrier blocking (
) afterwards.
Whiff Option Select
- XRD and +R Sol jumps towards an airborne opponent and presses ~
- If opponent gets hit/blocks j.
- then Sol will do j. > j.
- If opponent avoids j.
- then Sol will not do j.
This takes advantage of the fact that you can input the next hit even before the first hit touches the opponent. Since two attacks will come out on hit/block, you have much more time to Hit Confirm the situation and do different this on hit versus block.
Whiff Option Select 2
- BBCF Jin does a meaty ~~
versus Azrael - If Azrael blocks the
- Jin will cancel into and Jin can continue his block string. does not come out since it was inputed before hits the opponent and is ignored by the game.
- If Azrael attempts a reversal Panzer Strike
- Jin will whiff his and do , which will go under Azrael's Panzer Strike. Since whiffed, does not come out since it is inputted during 's recovery.
- If Azrael attempts a reversal backdash
- Just like above, Jin will whiff his and do , which will hit Azrael during the recovery of his backdash
This is a fairly complex example that is specific to a knockdown situation of Jin versus Azrael. Azrael's Panzer Strike is slow for a reversal and misses versus low profile moves like Jin's . Jin has enough time to whiff and go into , which will beat both Panzer Strike and backdash. While this option select will work against characters with slow backdashes like Azrael, this example is unique in that it beats both Azrael's backdash and his Panzer Strike.
The timing for this is a bit more difficult to do since you will need to time the to start immediately after if it whiffs.
Conclusion
Option selects can take on many forms and are a powerful tool since they can deal with multiple options at once. There are far more option selects than what has been shown here, and can be a very useful tool your arsenal. Next time you notice the opponent "guessing" right all the time, consider that they may not be guessing at all!