Yes, there likely is. The Naruto and One Punch Man fandoms are both quite large, and it's common for fans to create crossover fanfictions. These crossovers can be really interesting as they combine the unique elements of both universes, like Naruto's ninja skills with Saitama's one - punch power.
One popular storyline could be where the main character, who has the abilities of Naruto and Saitama from One Punch Man, finds himself in a world full of different female characters from both universes. He then has to navigate through various challenges while also dealing with the romantic interests of these female characters. For example, he might have to protect the village like Naruto, but also deal with the over - powered villains like in One Punch Man, all while the female characters are falling for his unique blend of skills and personality.
Well, Mumen Rider could potentially be in a comical sense in a One Punch Man harem fanfiction. Just kidding! But seriously, characters like Fubuki are prime candidates. She has a strong personality and her own motives which could easily be intertwined with romantic feelings for Saitama. Also, any new female characters introduced to the One Punch Man universe in the fanfiction could be part of the harem. They might be normal civilians who witness Saitama's heroics up close and become smitten.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
A common storyline involves a villainess falling for Saitama. Maybe a powerful female villain who was initially out to defeat Saitama realizes his charm and power in a different way. She then joins the harem and has to adjust to the other girls. This creates a lot of tension and humorous situations as the other members of the harem are suspicious of her motives at first.
Well, one common storyline could be about Saitama getting involved in various comical and heart - warming relationships within the harem. For example, different female characters might try to understand his overpowered yet simple - minded nature. Maybe a character like Tatsumaki could initially be frustrated by Saitama's nonchalance but gradually develop feelings. Another could be that Genos gets caught in the middle of all the harem chaos as he tries to protect Saitama from any potential harm caused by the female characters' over - enthusiastic advances.
I'm quite sure there exists a 'Naruto is One Punch Man' fanfic. The world of fanfiction is vast and full of all kinds of creative crossovers. Writers might be intrigued by the idea of bringing Naruto's ninja skills and the unique world of One Punch Man together. For example, how would Naruto react to the overpowered villains in One Punch Man's world? And how would Saitama view Naruto's determination to protect his village? These are the kind of things that could be explored in such a fanfic.
Tsunade from Naruto is also a popular choice. She is a strong and mature character, which provides a contrast to some of the younger female characters. In One Punch Man, Tatsumaki is another one that is frequently part of the harem. Her powerful psychic abilities and somewhat tsundere personality add complexity to the relationships in the fanfiction.
A plot could involve Saitama ending up in the Naruto world. He quickly realizes that the power levels here are different. He may get involved in the ninja wars, using his one - punch ability to end battles quickly. Naruto, intrigued by Saitama's power, tries to figure out how he can achieve such strength. Meanwhile, the villains in the Naruto world start to fear this new, extremely powerful intruder.
One popular storyline could be Saitama getting transported to a different universe full of harem anime characters. He might then have to deal with the various personalities and powers of these characters while still being his overpowered self. Another could be the harem characters being brought into the One Punch Man world and how they adapt to its unique setting and Saitama's laid - back attitude. There could also be a story where a villain tries to use the harem characters to distract Saitama from saving the world, but of course, Saitama is unfazed.