Although it has been done before, I believe that the OPM series started the trend of this type of writing. Experts used to advise against writing overpowered protagonists because they can become monotonous, but OPM defied this advice and did well. I have noticed that the author used several techniques to keep it interesting. One of them is focusing more on other characters. Another one would be showing that the protagonist has internal conflicts and mental dissatisfaction despite being physically overpowered. In addition, characters have unique abilities. Like the author brought martial arts, psychic powers, Samurai, monsters, cyborgs etc and combined almost all type fighting elements to the story. This variety of fighting style increases the chances of vs battles. Viewers will be interested to know "Who would win if a Samurai battles a Ninja or a martial artist battles a cyborg?"