The premise of One Punch-Man is that its protagonist Saitama is stronger than anyone, and the mysterious source of his power may have been already explained: the limiter break. Because the manga started as a parody of the shonen genre, the reason why Saitama is so powerful has never been explained. However, one explanation proposed in the story makes perfect sense in the context of the most recent developments in the manga.
https://screenrant.com/one-punch-man-saitama-strength-explained-limiter-break/