As far as I know, the OVA is based on books, which I've never read so I don't know how much is missing.I gotcha. I'm going to watch both regardless, but I wanted to make sure I wasn't missing something of the sort. I've put the game on hold to watch these and have some kind of idea of what's going on.
The first handful of episodes of the TV series covers some of the same story content as the OVA but more in depth, then it switches to a different band of heroes for most of the show.
When comparing the two, I do get the impression the final few episodes of the OVA are heavily truncated and events are mashed together; the TV series purportedly follows the books more closely.