Monster Musume no Iru Nichijou OVA

MonMusu OVA

Everyday Life with Monster Girls OVA

モンスター娘のいる日常 OVA

Bundled with the 11th and 12th volumes of the manga.

The OAD adapted the manga's chapter 17 from the fourth volume, where Miia, Cerea, Mero, and Kurusu go to the gym's pool.