1. Saitama was based off of Anpanman.
2. Saitama worked in a convenience store before becoming a superhero.
ONE confirmed this on One Punch Man: Hero Perfection Q&A and it was also shown in ONE’s picture dairy on his old homepage. There is a scene where a robber is pointing a knife at Saitama and all Saitama did was wonder where is the barcode of the knife. Saitama thought that the knife was one of the merchandise.
3. There have been many Mob Psycho 100 references in the One Punch Man series.
4. One-Punch Man started online, then moved to paper.
Most of us international fans were introduced to One-Punch Man most likely via the acclaimed anime series, but the story actually started as a webcomic. Created by the anonymous named manga artist ONE, the comic launched online in early 2009, receiving almost 8 million views by June of 2012.