This was something I thought up while watching through the Hero Academy anime with some friends. I'm not sure if its been posted before but here goes.

Basically the body of this theory is that in the OPM world mysterious beings have started to spring up seemingly out of nowhere. At first it was the odd isolated incident but as time progressed more and more "monsters" started to appear. Eventually the Hero Association was born and regular Joe's would join the ranks to fight the monsters, some even having special abilities like Tatsumaki. The way people talk about her abilities it is apparent that espers aren't an overall common thing but they still exist in other forms.

Eventually the "monster" problem gets further out of hand and more and more people start to "monterfy" although that's not what it is. That is the earliest days on quirks developing, strange abilities that manifest seemingly at random. At first they start to develop in people when they take part in an obsessive part of their lifestyles, like eating lots of crabs or something silly. Then the "monster cells" are discovered, these being the isolated gene that causes quirks to develop, the ingestion of which will cause a manifestation.

Then we come to Saitama, a regular guy who is a hero for fun, his power being limitless. Eventually he's gonna grow a bit older and as more and more people start to develop quirks he takes Genos's advice and takes an up and comer under his wing. He teaches them the way of his work out and they too develop his amazing power, they then vow to pass the teachings on when the time is right. They take inspiration from their masters penchance for finishing foes with one punch and in an effort to hide their origins they name their new "quirk" One for All.

Over time however the teachings are diluted and by the time All Might has the power inside him they are all but forgotten and he just thinks any old extreme workout will do. So his power comes no where near Saitama's original strength.

So that's about the size of it. Hope its coherent enough.

Edit: also this is a bit of a rushed version, so ifanyone has any questions I'll be glad to field them in the comments haha.