This OVA is an unaired episode 13 that was included on the Bluray release of the series. It’s listed as an OVA on MyAnimeList, and Special 1 on AniDB, so we just labeled it as an OVA.
One short note here about the episode title, Recorder and Red Randoseru. In Japan boys wear black randoseru backpacks, while girls wear red ones. Thus the title is alerting you that everyone’s been gender-swapped in this OVA.
Edit: And of course, since I last checked, MAL changed it to special. Oh well.