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. How soon a character will have access to their aerial options depends on the length of their jumpsquat. A shorter jumpsquat allows the player to access their aerials sooner, which is beneficial. 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. An example of this is that if a move deals 15%, it will have 28 frames of shieldstun in Smash 64 (30 in the Japanese version) but in Brawl, it will only have 5 frames of shieldstun.
The amount of time it takes to drop shield without any shieldstun also varies between games. It takes 7 frames to drop shield in Brawl and Smash 4, 11 frames in Smash 64 and Ultimate, and 15 frames in Melee. One exception with this, however, is with Yoshi prior to Smash 4. His shield drop is 15 frames long in Smash 64 and Brawl and 16 frames long in Melee.
When a character jumps out of shield in Smash 64, they are intangible for the first two frames of their jumpsquat. This can be used as a quick defensive option to avoid attacks, which can then lead into either an aerial or an up smash/up special. This was removed in all later entries, however. While Yoshi cannot jump out of shield in Melee he can still parry (which he also has access to in Smash 64')', which is a potent out of shield option.
Another change which occurred in Brawl is that players can now perform an up special or an up smash during the first frame of their jumpsquat. In Smash 64 and Melee, the player would have to jump from one frame before they could up special or up smash. In Brawl, however, the player can now perform an up special/up smash the same frame as their jump begins, effectively allowing every chracter to perform their up special/up smash one frame earlier OoS. As an example of the effect this has, Marth's Dolphin Slash comes out on frame 5 in all games. In Melee, it cannot be done until frame 6 OoS but from Brawl onwards, it can now be performed on frame 5 OoS.
Melee introduced perfect shielding (also known as power shielding). If an attack hits a shield during its first 4 frames of startup, a power shield will occur. This has three useful effects. The first effect is that the player takes no shield damage. The second effect is that it can be used to reflect projectiles which is very useful against projectile users such as Falco. The third effect is that it allows the player to use any attack during their shield drop animation (known as power shield canceling), which can allow for punishes which would otherwise not be possible (Yoshi cannot perform this in Melee and Brawl.) Power shielding lost the ability to reflect projectiles in Brawl and its window was reduced to three frames, although it now induces much less shield pushback. In Smash 4, power shielding now reduces shieldstun.
Ultimate also allows players to use their up smash or up special out of shield without dropping shield or inputting a jump first. This makes it considerably easier for players to perform these options, especially as quickly as possible. Additionally, perfect shielding has been reworked in Ultimate. A perfect shield is now performed if a player drops shield within 5 frames of an attack. When successfully performed, this enables the player to act 3 frames earlier (in addition to having access to their entire moveset), allowing for harder punishes. Yet another change in Ultimate is that all characters have greatly increased traction, which makes OoS punishes easier to perform, especially with characters that had poor traction in previous games.
Understanding what the best OoS option to use is requires extensive knowledge of frame data. The player first needs to understand what their fastest options are and how useful they are. The faster a move is, the more moves it can punish. The usefulness of the option depends on its range, strength and reliability. For example, while Zelda's up smash in Melee is a frame 6 OoS option (her fastest option by a large margin), it is rather useless due to its abysmal range and reliability; with the move failing to hit multiple characters and even if it does, the opponent can easily escape the move. Bowser's Whirling Fortress in Melee is also a frame 6 OoS option but it is much more useful due to its greater range, ability to KO, its intangibility frames during its startup (which allows him to plow through moves, even if the move is not a guaranteed punish) and the fact that Bowser can move around while using it.
Knowing how fast an option is depends on what kind of option it is. An up special or an up smash simply requires knowing the normal startup frames of the move (in addition to an extra frame prior to Brawl). An aerial requires the jumpsquat frames as well as the startup of the aerial. In Ultimate, all jumpsquats are 3 frames so for every character, the speed of an aerial OoS is 3 + the aerial's startup frames. Jumpsquat lengths in the earlier games however depend on the character, so the player has to memorise their character's jumpsquat frames to make the most out of punishing OoS. A shield drop simply requires the player to add the length of the shield drop (which is 7, 11 or 15 frames depending on the game) to the startup lag of the move. For shields grabs, the player needs to know the startup frames of the grab and in Ultimate, the players needs to additionally add an extra 4 frames onto the startup.
The second important thing needed to understand the best OoS option to use is knowing the frame advantage the opponent's attack has on shield. Frame advantage in this case is the difference (in frames) between when the attacker can perform an option and when the shielding opponent can perform an OoS option. Shield advantage is determined by how much shieldstun the move causes and how much ending lag the move has. If a move has high shieldstun and low ending lag, it will grant a greater frame advantage than a move with lower shieldstun and higher ending lag. If the attacker can act the same frame that the opponent can drop shield or perform an out of shield option, the attack is neutral (or +/-0 frames) on shield. If the shielding opponent can act before the attacker, the move grants frame disadvantage (or - frames) on shield. If the attacker can act before the shielding opponent, the move will grant frame advantage (or + frames) on shield.
Most attacks in the Smash Bros. series (at least after Smash 64), grant a frame disadvantage on shield. Moves with a greater frame disadvantage on shield can be punished by a wider arrange of OoS options. For example, if Mario is shielding and he is hit by an attack which is only -4 on shield, his only guaranteed punish is his Super Jump Punch (as well as Mario Tornado in Smash 64), as the opponent can shield any other attack he has or they can use a defensive option (such as roll, spotdodge, or an attack with intangibility, invincibility or super armor if the character possesses an attack with one of those properties) to avoid his grab. If a move is -30 however, Mario has enough time to drop his shield and hit his opponent with a forward smash (or any other attack he has). The shielding opponent should always go for the strongest punish they can. Using Mario shielding a -30 attack as an example again, while Mario can Super Jump Punch, he shouldn't use it as it will not KO the opponent until very high percents. If Mario uses a forward smash, it will KO the opponent if their percent is high enough. As a result, Mario should opt to use to forward smash as it will grant him a greater reward. He should only use Super Jump Punch for less punishable moves when it is either his only option or if he has no other more effective practical option.
If an attack cannot be punished out of shield, it is considered to be "safe on shield". If an attack is safe, the attacking opponent can either shield, use a defensive option or use an attack to shield, avoid or even beat any of the shielding opponent's OoS options. Whether a move is safe or not can be character dependent as some characters can punish OoS earlier than others. When a move is universally safe however in all games is when an attack is -2 (so the attacker can only act 2 frames earlier than the shielding opponent) on shield or less. In this case, the shielding opponent should either keep shielding or perform a defensive option. While the shielding opponent can still use an OoS option, it is riskier as it is not guaranteed to hit the attacker (who can then potentially punish the failed OoS option). If a move grants frame advantage (+ frames) on shield, the attacker can act before the shielding opponent can. If the attacker throws out an attack before the shielding opponent can act, it will hit their shield before they can use any of their OoS options. In the most extreme cases, this can lead into a shield break combo.
When frame data resources state the frame advantage a move has on shield, it makes certain assumptions. For grounded moves, they assume that the move is landed on the first possible frame. For aerials, they assume that you land the frame after landing the move (Smash 64 and Melee resources will also take L-cancelling into account in this situation). Because of this, an attack may be safer or less safe than what is commonly stated within an actual match. As a result, a move can sometimes be punished, even if it should be safe in theory. Additionally, an unsafe move can go unpunished if the shielding opponent mistimes their OoS option (which is especially easy to do in earlier Smash titles) or if the attack is landed during its later active frames, giving the shielding opponent less time to punish.
This concept is not exclusive to the Smash Bros. series as it also applies to any other traditional fighting game. The main difference between Smash Bros. and other fighting games however is that in other fighting games, the player has access to their entire moveset immediately after blocking while in Smash, the player has restrictions on what they can do due to how shields work (except after perfect shielding).
Examples of OoS moves