Spoiler Summary [Click to show or hide]:
OPM Manga Chapter 163.5 is a flashback of Saitama from one-and-a-half years ago.
He was doing push-ups and his daily exercise regime in a park before he got insanely strong.
While refreshing in a shower, he noticed his hair falling, which has already been happening a lot lately.
After touching the back of his head and realizing his scalp is sticking out more than usual, he frantically tries to look at it in the mirror by swiftly turning around, hoping to beat the mirror’s reaction time.