This manga, Nana to Kaoru and its sidestory, Nana to Kaoru Arashi, are the top two favorites out of all I have ever read. The mangaka, Amazume Ryuta-sensei, is an outstanding and prolific artist and author, one of my best mangaka IMHO, though in making a 24-minute long OVA was great mistake.
On a stand alone, one-shot OVA, it was fine work, I would guess, though leaving me wanting for more to follow. But to make such a short, one-shot episode based on the works published to date of release? Cruel and unusual punishment, there was nothing we, the audience and followers of Amazume-sensei and fans of the Nana to Kaoru series, did that deserved this form of punishment-game.
I do not know the minutia that went into the adaptation and manipulation of the storyline to this short episode, nor why it was a one-shot, though to make a stronger showing of the Nana to Kaoru universe, there is a crying out to make at least a 24-episode series.