This OAD, titled 'episode 12.5' will ship along the limited edition of the sixth volume of the manga by Shungiku Nakamura, along with the DVD that will come out in September.
Here's the promo, which lasts 17 seconds, is based on the story of Yoshino.
The series has had success and popularity among shonen-ai fans and it's one of the few series if its genre (if not the first) to make it mainstream, probably due to the wide bishonen appeal of all characters.