MyAnimeList sets this as episode 1 of the OVA, but this is more properly called episode 13 of Season 1. Season 1 including this episode covers volume 1 and 2 of the light novels. Hence, it is unfortunate that this is labelled a side story instead of a sequel as this OVA follows directly the events of episode 12. Remember that party scene at the end of episode 12? In the light novels at least (as I can't remember if it was ever mentioned in the anime), Hazuki has started to teach Miyo social graces and how to deal with people in parties. This isan informal party and should serve as practice for Miyo as she will likely appear in more formal parties later.

In the actual party, Miyo is apologetic for what she perceives as mistake. A lot of viewers might be annoyed with her, but you should consider that she has made an incredible progress from that person who had so low self-esteem. She is starting to spread her wings as the beautiful swan that she is instead of the ugly duckling she had thought herself to be.

Another highlight of this episode is the son of the Emperor, Kiyoka's childhood friend. He provided important information and apologised to Miyo for whatever unhappiness she had experienced.

And we also get to meet Hazuki's former husband and her son. He still have good relations with Hazuki despite their being divorced.

Overall, although there's nothing significant or emotionally gripping that happens in this episode, it is still worth watching. It's a cute episode, an episode of Miyo in bliss as the betrothed of Kudou Kiyoka.

If you take this into context that this is the actual last episode of the first season (the very last chapter of the second volume of the light novels), this episode is great, a nice pause before we venture into whatever drama season 2 is about to deliver.