Man, Eyeshield 21 is the reason I found it back around 2012! I was desperately looking for something that would be just as good (unfortunately, never found anything by ES21's writer) and lo'n'behold I discovered OPM! Best find.
Getting to see it animated was pretty great. Though honestly I didn't need it at all haha~
Saitama is the kind of protag i dig a lot - he doesn't give a shit about all the nonsense in the hero industry and does his own thing for his own reasons. The contrast between the lazy persona that no one but select few expect anything from and his true one-hit-kill badass self is mmmm delectable. Also hilarious.