Oneiric Diary is the third extended play by South Korean–Japanese girl group Iz*One. It was released on June 15, 2020, by Off the Record Entertainment. The album is available in three versions: Diary, Oneiric and 3D, and consists of eight tracks including the lead single, "Secret Story of the Swan".

Iz*One was a South Korean–Japanese girl group formed through the Mnet reality competition show Produce 48. The group was composed of twelve members: Jang Won-young, Sakura Miyawaki, Jo Yu-ri, Choi Ye-na, An Yu-jin, Nako Yabuki, Kwon Eun-bi, Kang Hye-won, Hitomi Honda, Kim Chae-won, Kim Min-ju and Lee Chae-yeon. They were managed by Off the Record and Swing Entertainment.