The Angel Sanctuary OAV is 3 episodes long. It is not, contrary to popular belief, a TV series, so those three episodes are, unfortunately, all that we will (probably) ever see. The OAV itself covers the first three graphic novels and part of the fourth, covering the entire first "arc" of the story. The characters featured are much less than the manga, as it cut out Kirie and Ruri and a few other less important characters. Even the game "Angel Sanctuary" was removed from the OAV.
I recommend the OAV, but it is a good idea to have read the manga beforehand. I enjoyed the OAV, but I think that it can be confusing if you haven't read the manga. The ending of the OAV is also pretty upsetting if you don't read the manga, because you are left on a cliffhanger that just lasts forever. Even I, who had already read that arc of the manga, wanted to see more, and unfortunately there is none.
The OAV leaps from plot point to plot point, cutting out everything else in-between or squeezing it in somehwere else. The comical moments that were featured in the manga have largely been taken out, and thus the anime feels a bit darker than the manga. The art is pretty standard as far as anime come, but the still graphics that appeared periodically throughout it were beautiful. The character designs were drawn by Kaori Yuki, and you can find those character sketches in the Lost Angel artbook or can visit the images section to see some of them.
The music for the OAV is superb. I bought the OAV soundtrack CD and there doesn't seem to be a single weak track on it. Most of the tracks are instrumental, except for the opening theme "Messiah" and the ending theme "Knife of Romance".
The seiyuu (Japanese voice actors) are awesome, they do a great job and they're all very well chosen for the characters their chosen for. Some voice actors are Koyasu Takehito as Kira and Yuji Ueda as Katou. I didn't really enjoy the English voice acting as much, but you can tell from the extra feature on the DVD that they did work hard on it, so I can give them that credit.
The DVD was released in America in July 2001. You can visit the official page
The dvd, though I have my problems with the dubbing and translation (They really could stop calling Alexiel a "broad" -_-), was done beautifully. They have wonderful extras, and I think the dvd is worth buying, even if just for the extras. I liked that they had a picture gallery that you could access with a dvd rom, and the Angel Sanctuary quiz was kind of fun. It even had questions that pertained to the manga, which is a special addition for anyone who has read it.
Please at least read a graphic novel or two of the manga before you see the OAV, or otherwise some things are going to be ruined for you. Or, even better, read the first four graphic novels! It's more fun that way.