Spoiler Summary [Click to show or hide]:
OPM Manga 20.5 takes us back to the young Saitama with his hair still intact…
After following three heroes, he was led to the Shelter with other citizens for evacuation. This was not his plan, as he thought following the heroes would lead him to the monster.
The monster defeated the three heroes, so they escaped to the evacuation center.
Saitama tried to get out to fight the monster but was prevented by the citizens. After two hours of agonizingly holding his pee, he broke the wall and escaped to a convenience store.
The 170,000-year-old Cicada Nymph showed up to attack him but was immediately destroyed by a single punch from the young and haired Saitama.
The scene cuts back to the present, where Saitama now has a suit with his hair bald while a new monster, the 170-thousand-year-old Cicada Imago, flies towards him to attack.