One Punch Man Fanfiction

Hrh-hrm. Waaaaaaan paaaaaaaaaaaaaaaawnch.

To start things off, here's The Strongest Hero, a story where Saitama is actually recognized for his deeds and Vaccine Man appears to actively fear him.

Enemy Number One, in which Saitama is the villain.

Minus Saitama, in which we get a look at the OPM world if Saitama weren't around.

And then there's Shopping Vibes, a crossover between RWBY and OPM that makes some... amusing notes about Saitama existing in the RWBY-verse.