So cringe at people calling him Saitama and OP with stupid plots and etc. Totally have short-term memory, bad attitude and illiteracy.
Since the start of the manga, we see mc trained to the point he actually died. He trained to punch in special space-time magic, which I assume something like 24 hours in there = 1 hours irl. Meaning he trained way more than 2 years.
During his fights in current time, he never used that 1 hit KO skill once.
And you know what, calling him Saitama isn't wrong. Because similar to Saitama who also trained himself to half dead so many times until he achieves that strength. Not some cheat power appears out of nowhere.