Where does Saitama's power come from and why does he have no hair? This is my theory.
Saitama decided to be a hero just for fun. As his hobby. This is a legitimate, pure-hearted reason, no matter how you look at it. The very same reason behind Blast had, who number occupies rank 1 in the Hero's Association.
Everything Blast did happened before Saitama started harnessing his power during his daily training routine. He has never showed up during OPM's adulthood. Could Saitama be Blast somehow? I don't think so.
It is well known OPM is a parody of the entire shonen anime genre, so maybe the secret lies within stupidly reversed similarities like everything else in the show. We have three main factors here: while training, Saitama started growing his powers while losing his hair. Harsh training, sudden powers and hair.
Hey remember that other anime where Izuku got his incredible powers from All Might? By eating a strand of his hair. Thanks to an extraordinarily straining exercises, his body managed to build up in order to handle the strenght of Earth's mightiest hero. We have everything in common so far: harsh training, sudden powers and hair.
That's, in my opinion, where Saitama must have come from: during his harsh training, there was a casual meeting between him and Blast, who ate a strand of Saitama's hair after a short conversation of why he wants to become a hero and realizing it's for the very same reason Blast has.
Just like a Dragon Ball fight lasts dozens of episodes while OPM fights almost always ends instantly, such an incredibly important moment would have been trivialized.
This encounter was very short. Saitama probably didn't even know Blast was a hero, much less the number one hero of all time. Blast must have died in combat after the fact, or retired, which explains his absence in the present time of the series.
Boku no Hero is also the other franchise where there is a hero association in the first place - further strengthening the potential parody of the main character is OPM as well.
