As always, I won't sum up the events but just give my personal opinion. The OVA doesn't put an end to the anime's ending (considering the manga wasn't finished right after this event) and just acts as something cool which was added to give me more screen time to the characters (in an ovbiously funny and soft way). So basically you will get to see every major characters + every minor characters (except a few) put all together in a canon story.
It should be seen after you watched the 12 episodes, so you can understand every aspect; without that, it might be a mess. There is no addition to the original plot but it remains very funny to watch, with a cool song at the end. I consider it as a "good bye" to the viewers, but not something radical (unlike Kill la Kill's OVA, which acted as a "no more screen time for the anime, thank you for watching and understanding"). To me, it would have been a decent transition between season one and two, to make people being patient... but there is no season 2 and there won't be any - 99.99% sure -. So yeah, I really think it should be taken as a friendly last screen time for the whole anime.
Don't be sad though, the manga - even finished nowadays - adds a ton lots of stuff. I started reading it after I watched the anime : highly recommended !
