Long ago, a man with a Quirk that could steal and distribute quirks, forcibly gave his seemingly Quirkless little brother a Quirk or "Superpower" that allowed him to stockpile power in his body. However, the little brother already had a Quirk with no effect other than that it could transfer a person's Quirk to another. The little brother's Quirk and the Quirk given to him by his older brother merged and became One For All.
One For All allows the user to access stockpiled power, which grants the user momentary superhuman strength at destructive levels, increased agility, increased hearing, and superhuman speed. It has not been shown if the Quirk that stockpiles power simply gains more energy as it is passed on (the first user could not beat All For One but recent users could which suggests a power increase) or if it is able to imprint other Quirk abilities as it is passed from user to user.
One For All receives its name from the fact that it can be "inherited" by others. Like a torch, it is passed on from user to user. The method of inheritance and transference of One For All is done by the recipient ingesting the current One For All user's DNA, as shown with All Might giving Izuku his strand of hair to ingest which allowed Izuku to inherit One For All. One For All will not transfer to a new recipient unless its user wishes it, thus One For All cannot be forcibly stolen. For example, the Hero Killer Stain ingested Izuku's blood, but did not inherit One For All because Izuku did not wish it. However, this does not mean One For All cannot be forcibly passed on.
Also, One For All seems to alter the user's appearance in different ways. For Toshinori Yagi, it significantly enlarges his muscles and overall body mass to make him look like a completely different person. For Izuku Midoriya, usage of One For All causes the empowered part of his body to glow, with vein-like lines crossing through each other. It is unknown what the usage of One For All looked like for its previous users, however.
After One For All is transferred to the recipient, the previous One For All user can still use the "leftover embers" of One For All, and thus can still access One For All's power. However, these leftover embers will not last for long, and the previous One For All will slowly run out of the embers, resulting in the previous user losing One For All completely. According to All For One, using these last embers continuously will rapidly increase the loss of the power. After All Might defeated All For One, the embers of One For All have burned out, and he cannot even maintain his muscle form for more than an instant.
Proper usage of One For All requires intense and rigorous training of the body to prevent the successor from being torn apart by the raw power. Even with the training, the Quirk still puts intense stress on the user's body (as when Izuku first used it, he broke his right arm and both legs).However, not every user is stressed the same, and some can get used to it rather quickly, as noted by Gran Torino, who went straight to actual combat training for All Might, as the latter did not take long to adapt to his then-new powers.
However, it is unclear what the true nature of One For All is. When Izuku was about to lose his match against Hitoshi Shinso, because of his Quirk, he feels the presence of multiple people coursing through his body and activated One For All to break free from Hitoshi's control. Izuku theorizes that since his Quirk is the "sacred torch that is passed down through the ages," those people that he felt were the lingering presence of those who had this power before him. He also mentions seeing someone with the same hairstyle of All Might among those who appeared in front of him, indicating that his presence is imprinted on the Quirk.
All For One's Brother
The first user of One For All was the little brother of a villain who had the Quirk, All For One. Though small and sickly, he had a strong sense of justice and did not back down from opposing his brother's evil deeds. One For All was formed from the mutation of the first user's innate Quirk, and the Quirk forced upon him. The first user attempted to defeat his brother with his newly merged Quirk (now called One For All), but was not strong enough. He then passed One For All down to a successor in the hopes that someone would eventually make it strong enough stop his evil older brother, a cycle that continues to this day.
All Might's predecessor and the seventh successor of One For All. Little is known about Nana's proficiency with One For All, other than she was not strong enough to bring down All For One.
As world-renowned hero All Might, Toshinori is the 8th owner and a very proficient user of this Quirk. He is capable of controlling his own strength and focusing his power into any extremity he chooses without recoil. He is probably the best-known user of the quirk, if not one of the most talented, as noted by Gran Torino that Toshinori did not take long to get used to One For All. However, due to an untreatable wound that was inflicted on him, he could only utilize his power for three hours straight; any longer and it would strain his body. And if forced to use his Quirk more than he should, he may shorten the time limit for using his Quirk as a repercussion. After transferring the power, his ability to use One For All slowly diminished as the leftover embers were all he had to use as All Might, and after his battle with All For One, Toshinori completely depleted all the embers of the torch which is One for All. Although he can still transform for an instant, he cannot maintain his muscle form anymore and would revert back very quickly.
Izuku is chosen by All Might to become the ninth holder of One For All. Initially, Izuku could unleash powerful displays of strength at the cost of injuring his own body. Despite training months before gaining this power, One For All's physical backlash was the first step Izuku needed to overcome in mastering this power. When Izuku manifests One For All's power, red vein like streams surge throughout his body. He first used One For All's full power to destroy a giant villain bot in a single punch at the cost of breaking his arm and both his legs.
Izuku eventually develops a way to minimize damage to himself by focusing One For All's power into his fingers individually. He takes this a step further when he subconsciously holds back One For All's destructive power when using it on a living person. From this point on, Izuku began learning on how to properly modulate the output of his Quirk more effectively.
By focusing One For All in certain portions of his body, he could use the Quirk's power to a lesser degree to avoid breaking his bones. He showcased this in both encounters with Shoto Todoroki during the U.A. Sports Festival. He was able to focus One For All into his hand to break Shoto's defense during the Cavalry Battle, and also to punch him a few times during their match in the finals. All Might believes at this point Izuku is only able to harness five percent of One For All's power without backlash. Izuku's fight with Shoto proved he was okay with injuring himself as long as he believed he could be healed later on by Recovery Girl. After this battle, Recovery Girl refuses to heal self-inflicted injuries of that nature and demands Izuku find a new way to use his Quirk.
It is only after training with Gran Torino that Izuku discovers a way to use One For All with no repercussions. During their time together, Gran Torino taught Izuku that his movements and smashes were too slow whenever he would summon his Quirk's power. While training himself in an alleyway, Izuku tried regulating his Quirks power into his legs in order to scale the side of a building. Unfortunately, he discovered he was too slow to summon the power when he needed it. The culmination of his early training during this internship taught Izuku that he using One For All only in the limbs that he needed it, thus severely slowing down his reaction time. To compensate, Izuku developed One For All: Full Cowling, by using One For All's power flowing continuously throughout his entire body. This negates Izuku's need to "turn on the switch" whenever he needs to use his Quirk. One For All: Full Cowling takes advantage of the five percent Izuku can safely use to increase his strength and agility.
Using Full Cowling, Izuku could easily scale the side of buildings, jump large clearings between structures and quickly navigate through cluttered areas. Many of his newfound movements are attributed to watching the way Katsuki Bakugo propels himself using his Explosion Quirk. Using Full Cowling, Izuku could spar close to the speed of Gran Torino and even held his own against Hero Killer: Stain in his first real fight. Even so, if Izuku does not remain calm and alert while using his Quirk, he can go over his five percent limit and injure himself.
During the rescue training race, Izuku used his full cowling to enhance his speed and agility and was able to outpace his classmates. This display garnered much shock from his classmates and All Might.
In his second fight with Katsuki, Izuku became able to use One For All at eight percent of its full power without repercussions. Izuku can access any percentage of the full power of One For All, however, any higher than his current limit will damage his body as seen in his fight with Overhaul.
While in physical contact with Eri, Izuku was temporarily able to use One For All at 100 percent of its full power with no consequences, because Eri's Quirk was able to constantly rewind all of the damage Izuku was doing to his own body.
In his training sessions with All Might, Izuku revealed to All Might that he had discovered during the internships he was able to use 20 percent of One For All for a very short time if in a desperate situation. Any longer than a brief instant would cause harm to his body. Lamenting the his "inability" to perform any long ranged attacks in the battle, All Might tells Izuku that he is capable of using long ranged attacks. All Might had planned to wait until Izuku was able to use at least 15 percent of One for All to begin teaching Izuku, but Izuku already became capable of handling the technique to create wind pressure with his body and decided it was time to teach Izuku how to harness the brief moment he is able to bring out 20 percent of One For All to perform long-ranged wind pressure attacks without breaking his body.
Most of the attacks used by Toshinori are based on the names of places in America. Even the training schedule he made for Izuku was named such.
- Texas Smash (テキサススマッシュ Tekisasu Sumasshu?): Toshinori throws a right punch with enough force that the wind pressure sends most things flying away. This move is first used to save Izuku from the Sludge Villain.
- Detroit Smash (デトロイトスマッシュ Detoroito Sumasshu?): Toshinori performs that same attack as Texas Smash except this move is with a downward punch instead of a straight punch. The punch is powerful enough to create a shock wave large and powerful enough to clear the weather. This move is first used to save Izuku and Katsuki Bakugo from the Sludge Villain.
- Missouri Smash (ミズーリースマッシュ Mizūrī Sumasshu?): Toshinori runs towards an enemy while keeping his hand flat and hits the enemy on the head while running past them. This move is first used against Trapezius Head Gear.
- Carolina Smash (カロライナスマッシュ Karoraina Sumasshu?): Toshinori runs towards an enemy while keeping his hands in a cross position, then in a downwards cross chop to the enemies head. This move is first used against Tomura Shigaraki, but it is intercepted by Nomu, who withstood the full force of the attack, seemingly unharmed.
- New Hampshire Smash (ニューハンプシャースマッシュ Nyūhanpushā Sumasshu?): Toshinori blasts himself towards his enemy and smashes his body into the enemy, crushing and injuring the enemy with his great weight. This move is first used to propel himself at Izuku.
- Oklahoma Smash (オクラホマスマッシュ Okurahoma Sumasshu?): Toshinori whirls around while enemies have latched onto him, spinning with enough force that when they are released they are easily thrown through concrete. This move is first used against Nomu.
- United States of Smash: Toshinori's final and strongest move. Using the last remnants of One For All left in him, Toshinori delivers a massive punch with enough force to reverberate shockwaves. This unstoppable attack was used to defeat All For One and marked the end of his hero career.
- Detroit Smash (デトロイトスマッシュ Detoroito Sumasshu?): The same as All Might's move, except Izuku sometimes does it in the form of an uppercut, creating an air pressure blast.
- Delaware Smash (デラウェアスマッシュ Derawea Sumasshu?): Izuku flicks his finger to create a powerful shock wave. Doing this fractures said finger, limiting the number of times he can use this move without gravely injuring himself to 10 times.
- One For All: Full Cowling (ワン・フォー・オール フルカウル Wan Fō Ōru Furu Kauru?, Viz name): This technique allows Izuku to activate 5%, and later 8% of One For All throughout his body, letting the power course and flow through him instead of concentrating it in one location, and negating the need to "turn on" the power every time he needs to use it. Activating this technique gives him enhanced strength, speed, mobility and agility. It also prevents him from breaking his bones when attacking.
- 5% Detroit Smash (5 パーセント デトロイトスマッシュ 5 Pāsento Detoroito Sumasshu?): Izuku activates One For All: Full Cowling, then jumps into the air and delivers an enhanced downward punch to the opponent. He has also used this move in the form a straightforward punch.
- Delaware Smash Air Force (デラウェアスマッシュエアフォース Derawea Sumasshu Eafōsu?): Izuku uses his upgraded gloves to direct a blast of compressed air against his foe, possessing enough concussive force to stun Gentle while he was in mid-air, giving Izuku an opening to catch up to him. By using all five of his fingers, he can launch up to four Delaware Smashes simultaneously.
- One For All: Full Cowling - Shoot Style (ワンフォーオールフルカウル シュートスタイル Wan Fō Ōru Furu Kauru Shūto Sutairu?): A fighting style of using One For All that deviates from All Might. Izuku realizing that just imitating All Might's style which has its special moves centered on his fists was not the best style for him and was actually causing more harm to his arms, he developed a new style that centers more on his legs with the help of his friends. Shoot Style concentrates One for All: Full Cowling's effects in his torso and legs rather than his arms, allowing him to put out even more power due to the fact that the human leg muscles are naturally stronger than the human arm muscles, granting him more power and stability and allowing him to step away from the shackles of having to imitate All Might.
- Shoot Style: St. Louis Smash (シュートスタイルセントルイススマッシュ Shūto Sutairu Sento Ruisu Sumasshu?): While using the Shoot Style, Izuku leaps into the air and before swinging around to deliver a roundhouse kick to the opponent's face. Izuku used this move to incapacitate Gentle Criminal, who had been empowered by La Brava's Lover Mode Quirk to match Izuku's use of One for All: Full Cowl at 8% of its power.
- One For All: Full Cowling - 100% (ワン・フォー・オール フルカウル 100
%Wan Fō Ōru Furu Kauru 100 Pāsento?): Izuku activates One For All to its fullest extent throughout in Full Cowling, granting him a tremendous boost in speed and strength that allowed him to easily overwhelm the likes of Chisaki. This is first used against Kai Chisaki to protect Eri. The power increase seemingly gives him fire-like trails coming from his eyes and light colored hair. He can only use it infinitely with no damage done to his body while in physical contact with Eri because her Quirk constantly rewinds his body to a previous state.
- One For All 20%: A technique that allows Izuku to use 20% of One For All throughout his body, it causes a lot of pain to the point that he feels as if his bones are about to break.
- One For All 100%: A technique that allows Izuku to attack using the full power of One For All in one concentrated part of his body. After initial use, the limb breaks instantly. He can still use expended limbs in combat, but it causes scarring once they're completely healed. 
- 100% Detroit Smash (100 パーセント デトロイトスマッシュ 100 Pāsento Detoroito Sumasshu?): While using 100% of One For All, Izuku strikes his target with a lunging punch.
- 1,000,000% Delaware Detroit Smash (1000000 パーセント デラウェアデトロイトスマッシュ 1000000 Pāsento Derawea Detoroito Sumasshu?): Izuku activates 100% of One For All during an adrenaline rush and then strikes his opponent by first using a Delaware Smash and then a powerful Detroit Smash, hence the Delaware Detroit Smash. During the climax of his battle with Muscular, Izuku manages not only to overpower but also to launch his augmented opponent with enough force to create an impact crater in solid rock with the strike alone.
- To date, most of the moves using One for All are named after states and cities in the USA (e.g. Missouri Smash, Delaware Smash). This is consistent with the manga artist's desire to model All Might after American comic book heroes, especially in All Might's hero form.
- According to All Might, to activate One For All, the user must clench their buttocks and scream within their heart: "SMASH!".
- So far every named user has had a number in their name somehow correlating to what number user they are. The 7th user is Nana which is Japanese for 7. Toshinori Yagi has the kanji for 8 in his name. One of the kanji in Izuku's name can also be read as "kyu".
- One For All's name is an inversion of All For One, showing the differing philosophies between their wielders. One For All showing "One Quirk for multiple people" (through passing it on) and All for One showing "All Quirks for one person".
- One for All, together with its counterpart, All for One, may be derived from the popular motto of the titular heroes of the novel The Three Musketeers by Alexandre Dumas.
- It is unknown if One For All can be passed back to a previous user if the current user allows it.
- As the truth behind One for All is only known to a select few, Izuku's Quirk is listed as "Super-Power" in UA and other records.