Sekaiichi Hatsukoi OVA
Sekaiichi Hatsukoi Episode 0
Sekai-ichi Hatsukoi: Onodera Ritsu no Baai
Sekaiichi Hatsukoi: Yoshino Chiaki no Baai
Sekai'ichi Hatsukoi
世界一初恋 OVA
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.