According to Wikipedia:
[The OVA is] based on the Read or Die manga series... [It] is a continuation of the Read or Die storyline, taking place a few years after the events of the manga.
R.O.D the TV is a 26-episode anime TV sequel to the Read or Die OVA
Incidentally, R.O.D. started as light novels and manga published at the same time.
As with all OVAs I'...
You can watch any anime and its OVA(s) without reading the manga. However, usually manga do contain extra content that aren't covered in the anime and OVA(s). So while you don't need to read the manga to watch the anime, you can for an extra bit of info/story.
So you could read the manga on top of watching the anime and OVA(s) for Read or Die, but it's not ...