A monster is moving toward City Z, one that has been created by the mysterious organization known as The House of Evolution. Genos, a young cyborg, attempts to stop it, but gets in over his head before Saitama happens along.
Saitama and Genos glean information about The House of Evolution and its founder Dr. Genus, a scientist with dangerous ideas on artificial human evolution. Meanwhile, Dr. Genus learns of Saitama's amazing strength and has plans of his own for One-Punch Man.
A group of battlesuit-clad terrorists called "The Paradisers" appears in City F, led by an enormous man known as Hammerhead. Calling for the redistribution of wealth, the group targets the fat-cat Zeniru, who in turn dispatches his bodyguard to defeat them. Unfortunately, Saitama is on the same errand.
Though he's been doing the hero gig for three years, Saitama realizes that the public knows nothing about him. Determined to get a little recognition, he decides to become a pro hero, but first he must pass the Hero Association tests and become a full-fledged member.
It's been five days since Saitama became a professional hero and he has yet to do anything heroic. When Genos informs him that C-class heroes who don't fight bad guys are dropped from the hero registry after a week of inaction, Saitama heads out on patrol, yet can't find any criminals or monsters to stop.
An enormous meteor hurtles toward City Z, forcing the Hero Association to ask its S Class heroes to respond. When all their efforts prove in vain, Genos decides it's up to him to protect the home of his master and stop the meteor, no matter what the cost.
As the battle with the Deep Sea King reaches its climax, more heroes fall to the aquatic despot's brutal attacks. Which of them has what it takes to be a true hero?
Saitama tags along to an emergency meeting of Class S heroes and discovers that a seer has foretold a serious danger threatening the Earth. Though the crisis could occur anytime in the next six months, the heroes are surprised to hear the roar of explosions that very hour!
As a colossal spaceship floats in the skies above City A, a member of its monstrous crew battles on the ground against a group of S Class heroes. Despite their powers, they can't seem to finish it off. Meanwhile, Saitama has boarded the enemy ship on his own.
The inconceivably powerful Lord Boros has traveled untold distances in search of someone who can cure his boredom: Saitama! With the fate of humanity in the balance, will the two champions at last find in each other the worthy opponent they've been searching for?
During a coincidental train ride, Genos and Fubuki are put to the test when an anti-Hero Association group plants a bomb on the train. *This OVA is half as long as a regular episode.
A story about Saitama from when he still had hair and his only costume was a track suit. Several shops in Saitama's neighbourhood have gone out of business and the owner of the last one left needs his help.
A story set sometime between Mosquito Girl's defeat and Genos showing up to Saitama's apartment. Genos wants to learn the secret to Saitama's power and decides to follow him and to observe everything he does. At the same time, Saitama is convinced that someone is stalking him.
After the events of episode 7, Bang begins to spy on Saitama and Genos to gauge their abilities. Hopeful that his Water Stream Rock Smashing Fist martial arts is stronger, and that proving so will coerce Saitama and Genos to study under him, Bang invites Saitama and Genos to his dojo under the pretense of giving them free meat.
The Heroes of the Hero Association goes to an "onsen" ret.eat unwind after the battle with the alien invaders, with a mystery afoot when one of the heroes get stabbed...Child Emperor investigates the murder of Zombieman.
One Punch Man 2nd Season's commemorative recap special.