OS = Option Select
Option Selects are you doing moves that the game will automatically select the best choice for you
For instance, many characters can do Option Select jumping HK to Dragon Punch after they knock the opponent down by pressing HK in the air and then doing the Dragon Punch fast (before you hit the opponent normally).
What happens can be anything of the following:
1) If the jumping HK hits or is blocked, nothing happens (you can follow up with other attacks)
2) If the jumping HK misses because the opponent backdashed, the Dragon Punch will come out to catch them
3) If the jumping HK misses because the opponent did their own invincible attack, the Dragon Punch will come out and try to beat their attack
4) If the jumping HK misses because the opponent does Ultra or Super, nothing happens
All these 4 situations will be covered by doing that Option Select. There are many option selects in the game.
Another common one is Option Select crouching MP/MK/HP/HK and throw when blocking. What you do is hold down back, and press LP + LK + MP/MK/HP/HK. What happens is this:
1) If the opponent combos, nothing happens and you keep blocking
2) If the opponent breaks combo to throw, you will tech the throw (because you pressed LP + LK)
3) If the opponent pauses to try and bait you, you will throw out your MP/MK/HP/HK move
4) If the opponent does Ultra, nothing happens (you will continue blocking if you hold downback)
5) If the opponent does an invincible to attacks command throw, you will get thrown
So this Option Select covers 4 out of 5 situations (might be more). Option Selects can be useful for many situations where you have to guess what to do.
I hope that explanation was good for you, it's tricky a bit to explain.
very detailed....
usually i OS with lp+lk only...if the opponent try to bait, the lk will come out...and it it hits, can continue with further clk for some combos....but usually i fail la....