Spoiler Summary [Click to show or hide]:
OPM Manga Chapter 200 shows Saitama accepting Genos’ request to become his sensei and he promises him that he will teach him everything that he knows to become strong…
Genos visited Saitama’s apartment to formally request to become his student. Monsters suddenly appeared but Saitama handled them like they were nothing. One of them even destroyed his ceiling.
The Mosquito Lady was actually an experiment by a scientist and after seeing how Saitama beat her, he wanted to take blood samples from our one-punch hero, thus the appearance of monsters in his apartment.
The chapter ended with the appearance of a fat robot.