The character design of Saitama is based on the gag anime superhero Anpanman from Soreike! Anpanman (1988).
If you look closely, you'll also find manga related to ONE's other series, Mob Psycho 100, from time to time.
Saitama's name comes from the Japanese province where the creator "ONE" used to live.
Saitama reads a lot of manga.