Fair warning; if you are not up to date with the Manga please do not read. This article would include spoilers.
Saitama, the protagonist in the hit series One Punch Man, is known for three things; his memes, his ridiculous strength and finally, his lack of hair. Being the strongest character within his series, Saitama has pulled off many incredible feats and saved the planet more times than Team Rocket has failed to capture Ash’s Pikachu. However, this chrome-domed cape crusader was not always as powerful as he currently is. In fact he was a weak human with hair and was looking for a normal salary job. With the life drained from his eyes, Saitama just happened to stumble upon a monster that was about to kill a little boy over a harmless (but funny) prank. With the fire back in his eyes, Saitama fought and won against this lobster monster thus saving the little boy (whose father created the Hero Association thereafter thus proving that Saitama is the reason that the Hero Association exists). However this isn’t Dragon Ball Z or Naruto, Saitama didn’t awaken some hidden power to win the fight, and he was still a weak human that used his wits and guts to prevail.
So the question still stands; how did Saitama become so powerful?
Well throughout the series, this question has been asked several times. Saitama usually gives the same answer, but during his conversation with Genos within the House of Evolution, we got a little more insight into what Saitama believed to be his source of strength. According to him, a daily regime of 100 push-ups, sit-ups and squats along with a 10KM run is the key to achieving his strength. He further goes on to explain that he never used the AC or the Heater, no matter the weather to strengthen himself and to never take a rest day. This of course is denied by everyone there especially Genos who tells him that his training was basic (which shows just how normal Saitama was).
So if Genos is correct and Saitama’s training isn’t enough to provide him with the strength that he possesses; then how did Saitama become force of mass destruction that he is now? The answer is simple: Monster Cells.According to the Manga, these can increase your overall abilities and increase your lifespan.
However everyone that we saw that ingested it, gained the appearance of a monster thus disproving this theory. That was, until we met Hayate of the Wind and Flame of the Hellfire. During their conversation with Sonic Speed O-Sound, we came to understand that if one has full control of their body, one can retain their human shape while still gaining immense power. If we were to apply this knowledge to Saitama, it would explain how this Average Joe gained such immense power, and ironically, his major transformation was a permanent loss of hair (lol), which, to be fair, isn’t a bad trade off.
Perhaps his training did reduce the effects of the Monster Cells, or perhaps he ate it gradually to save money and that caused him to be affected differently. Well whatever it is, until we get a full reveal, this theory is my best bet at Saitama’s powers.