The line comes from Saitama, the protagonist of One-Punch Man, the anime and manga series about a hero who defeats any opponent with a single punch. His flat, unbothered personality became a central joke of the show, and short clips of his dialogue spread widely on soundboards, TikTok and YouTube edits. This 'okay' works because it captures that whole character in one word, so creators keep reusing it for deadpan reactions long after the original scenes aired.