I wouldn't go out of the way in trying to get the OVA. nothing in particular is explained, in my opinion. I'm not saying by any means don't get it and if you come across a copy, I advise you to watch it just for comparison, but in my personal opinion, it's not a must have to be able to understand Blood+
They'll probably cover everything that they say in the OVA about Saya in the series. And a 45minute OVA compared to a whole series that already lasts an hour with the 3 episodes already realised really isn't worth it.