So, I watched both OVA's and now can write a review on it. In short: the first is about Chihiro and how he had found a book about resurrection; the second is about Aria and her strange dream (I can't write what was this dream about because it will be a great spoiler for you; for same reason I'm not writing who is Chihiro&Aria). Story: First OVA is like a prequel for the TV series. If you didn't watched "Sankarea" yet, I strongly recommend you to start with this OVA -- you'll free from several question and won't say "WTF?!" after the first TV ep. 10/10. Second is sostrange and weakly connected to the main story. That is why I recommend you watch this after the TV series, but not the right after! 6/10.
Overall: 8/10.

Art:
Both OVA's are well-animated and have cute scenes. Quint essence of the TV series...
Nevertheless, I can't say that it's a masterpiece.
9/10.

Sound:
Just like I said about art. Also 9/10.

Character:
Well, well. All characters, except Aria in her dream, are shown true. True, but not great.
8/10.

Enjoyment:
7/10, also due to better first and worse second OVA.

Overall:
8/10. Very, very good additional episodes of "Sankarea". I like it.