Out of shield
Out of shield (abbreviated as OoS) options are any moves or other actions that can be performed while a character is shielding. These moves are usually used in response to shield pressure from an opponent. Since only some actions can be performed out of shield, the player who is shielding has fewer options both for fighting back and for movement. A character's set of out of shield options consists both of "built-in" moves from shield such as grabs and rolls, as well as many other moves like wavedashes, aerials, and up smashes. Moves that have high ending lag or very little range are easiest to punish by performing a move OoS.
Of all of the options that a player has while shielding, the most important option is jumping. Players can cancel the startup of the jump with either an up smash or an up special. Any aerial can be performed immediately after jumping, or the player can wavedash or simply jump away. While all up smashes and up specials are technically out of shield options, as they can be used while shielding, not many of them are effective. For example, Falco's Fire Bird in Melee can be used directly from shield, but is essentially useless due to its lack of hitbox while charging. The same goes for many characters' rolls or grabs, especially if they suffer from long lag. The quicker a move and the better its range, the better the move is OoS.
Starting in Melee, characters who can jump out of shield can also jump during their guard-off animation, providing all the same options as jumping OoS.
Differences between games
Between the different games in the Smash Bros. series, the amount of shieldstun has varied. As shieldstun disables a player's ability to use any of their out of shield options, the more shieldstun a game has, the more difficult it becomes to get out of shield. It is easiest to OoS in Brawl both because the amount of shieldstun is lower than in the other games, and because it takes only 7 frames for a player to drop a shield, making it viable for a player to drop their shield and then use any move. Conversely, it is the hardest to use OoS techniques in Smash 64 because of its high shieldstun.
Examples of OoS moves