The OVA "follows" the game. It's quite different from the game though, but if you're open-minded about it you can enjoy it just as much as the game itself (and hell... the characters are animated, which is why I really enjoy the OVA, on top of other things such as the music). The manga is closer to the game itself, but it still have some differences for the same reason (lack of space). However, both of them do have some aspects that weren't really delved upon in the game, such as Zelos and Regal's past. The Manga delves more into Zelos, and Regal has his own episode to himself really in the OVA.
Basically, the game obviously tops all, but there are aspects of the Manga and OVA that people can argue is better than the Game. (There are also the drama CDs, but I never listened to them woe is i).
or just go with what Erica said, because that's basically what I said but in much simpler terms. xD
... You haven't Erica? Where have you been. xD The third part is releasing next month, and I have a feeling that these days two episodes are going to be amazing. <3 If you haven't seen about ten second of Mithos, I don't think you're caught up in Tethe'alla yet. :P