Two OVA episodes featuring additional stories. Episode 0: Ritsu and Takano used to date back in high school, but broke up due to a misunderstanding. This is that story. Episode 12.5: Hatori and Chiaki go to visit the latter's family. It's really awkward in many, many ways.
Watch Sekaiichi Hatsukoi OVA episode 1 online — subbed and dubbed, with automatic AniList / MyAnimeList progress sync.