While our favorite band of survivors from the series does make an appearance in the OVA (they don on swimsuits too and scour a somewhat marooned island) the storyline here has little relevance to the Highschool of the Dead main story. I'm not going to give any details out, but you can expect a heaping helping of ecchi, though there isn't really any "full" nudity (and thus no censors).
Personally (and I repeat, PERSONALLY) I found the OVA to be tedious and a little boring compared to the main story frame. Basically ridiculous actions lead to ridiculous delusions which in turn lead to ridiculous ecchi scenes. Again, no connection to the brain bashing, zombie milling storyline in the prior 12 episodes.
I'd say, take a miss with this. Nine bucks just isnt worth it. Not sure why the OVA would be sold by itself, let alone who is going to pick it up by itself.... Unless the cover art appeals to you as a collectors item (as of current Amazon.com doesn't display, but I caught it in a Tokyopop news mail). If you're going to purchase any HOTD Dvd/bluray at all, I recommend buying the series as a whole.