Spoiler Summary [Click to show or hide]:
In OPM Manga Chapter 18, Saitama is chilling in his home with a manga on a warm sunny day when Genos appears with a huge backpack behind him…
He drops the backpack and tells Saitama Sensei that he wants to live with him to which Sensei quickly disagrees.
But after dropping a fat cash in front of him as a payment for rent, Saitama changed his mind and agreed.
Genos started taking down notes on Saitama’s daily training and exercise which is just his personal daily living routine. Saitama gets stressed thinking about training advice since he can only give muscle training
Then Genos informed his Sensei that a Class C hero gets delisted from the Hero Association after a week of no activity.
Saitama immediately hits the road looking for monsters where he meets with Speed-O’-Sound Sonic the next day.