One idea could be that Saitama from One Punch Man somehow ends up in the world of Overlord. He might be mistaken for a powerful player at first. Since he's so strong, he could easily defeat many of the powerful monsters in the Overlord world, which would cause a stir among the various guilds and NPCs. Another plot could involve the characters from Overlord trying to understand Saitama's power, which is beyond their comprehension as it's not based on magic or typical RPG stats.
A plot could be that the heroes from One Punch Man and the powerful beings in Overlord form an alliance to face a common threat. For example, a new entity that is even more powerful than the strongest in both worlds. The heroes and the Overlord characters would have to learn to work together, despite their different power systems and personalities. Saitama could be the key to their victory, but they first need to figure out how to use his power effectively in a coordinated way.
Perhaps in the fanfiction, a character from Overlord, like Ainz Ooal Gown, decides to visit the world of One Punch Man out of curiosity. He is shocked by the simplicity of the world's power structure compared to his own. Meanwhile, the heroes in One Punch Man are initially suspicious of this strange, powerful being. As time passes, Ainz starts to interact with the heroes, and they begin to learn from each other. Saitama might be the only one not overly impressed by Ainz's power, which could lead to some comical and interesting interactions.
Chapter 92 is the strongest chapter-level answer because it starts from Jiang Li, lightened from any task, had plucked a few leaves from the... and follows through on Finally, someone entrenched in the circle couldn't hold back and sent a....
A practical answer is Chapter 92, because it turns the idea into a concrete scene. The chapter opens with Jiang Li, lightened from any task, had plucked a few leaves from the..., adds pressure through Just as Jiang Li was about to refocus and continue expanding her..., and ends on Finally, someone.
The best chapter pointer here is Chapter 121, "Jiang Li Gains 5 Points for His Meritorious Rescue!". It opens with Seeing that Wen Jiaojiao's condition had improved significantly, Director... and closes on You're supposed to be a man who conquers mountains!] Experienced and....
If you want the clearest chapter reference, go to Chapter 141. Early on, It had happened in the afternoon when Aunt Ying's son and daughter heard...; later, the chapter leans on They went straight to Jiang Li's door and insisted on thanking her..
If you want the clearest chapter reference, go to Chapter 80. Early on, Jiang Yan and Wen Jiaojiao, one so smug he was almost ascending to the...; later, the chapter leans on And this was exactly the scene Director Sun wanted to see..
The best chapter pointer here is Chapter 82, "Jiang Yan and the Chicken Go Viral!". It opens with Jiang Li gave her a fleeting glance. and closes on And so, aside from Wen Jiaojiao, the remaining five set off, each....
If you want the clearest chapter reference, go to Chapter 96. Early on, Jiang Li's words seemed absurd, yet there also seemed to be a trace of...; later, the chapter leans on Her hand holding the phone began to tremble uncontrollably..
Look to Chapter 93 for the clearest answer. It begins with Jiang Li looked back and saw several aunties linking arms, walking up the..., keeps building through The leader was indeed the aunt to whom she had read fortunes this morning., and lands on I did the same when I was young, and when my dad.
If you want the clearest chapter reference, go to Chapter 123. Early on, After He Yun also carefully rolled up her trouser legs and walked down...; later, the chapter leans on [Jiang Li, why are you still wandering around the edge?.
The best chapter pointer here is Chapter 62, "There is Something Strange". It opens with Sitting on the edge of her bed, she pondered for a while before lifting... and closes on Jiang Li reached in, pulled out a piece of paper, and slightly raised her....