Do Mob and Saitama live in the same universe? In a former question it was pointed Mob appears in a cover photo of a One Punch Man manga. Does Mob appear in One Punch Man? . But also apparently Tatsumaki appears in a Mob Psycho manga (I've seen the anime but I havent read the manga), in an apparent scene of the manga itself, not a cover Tatsumaki in Mob Psycho 100

Do Mob, Tatsumaki and Saitama live in the same universe/planet kinda like Goku and Arale from Dragon Ball / Dr Slump ?
The answer is NO. 

Saitama's world and Mob's world is totally different. We can see that the art style of the anime and the main character is almost alike that's because they were written by the same creator, ONE. 

Mob appear in One Punch Man world.

(Image credit to @Pablo.)

Saitama appear in Mob Season 2 Opening. (Watch the cellphone, it's Saitama outfit.)


So why does these 2 not related story will appear on each other scene, I guess it was just a purely an advertisement purpose.