This OVA should only be watched by FGO players. If you are an NA player, NOW is the perfect time to watch this as it ties into the next big arc of the game! I suppose if more FGO gets more adaptations, anime watchers may end up watching this as well as part of the complete story. It's Christmas time, and as everyone is celebrating and preparing for the new year, our friendly staff at Chaldea are waiting, tense, about what happens next to the organisation. With the Animusphere family going kaput, a takeover is happening and no one knows what's going to happennext...

Story consists of heavy exposition, as a set-up for the next arc and a wrap-up of the singularities.
Art & sound are the usual.
We see more character building.

As a completionist, I encourage you to watch this if you are a FGO player.
"ap1001" wrote up a solid summary/explanation within his review, which I also recommend reading after watching the OVA.