I have this problem too.
I bought OWB, and from the looks of it... It only downloaded the sounds?... Not the ESM, meshes, textures or anything like that. The only OWB-related file in "Data" is a BSA file called: "OldWorldBlues - Sounds"
Try validating your game files (right click NV, properties > local files > Verify Integrity of Game Cache). That's what I did and now Steam is "re-aquireing" a 2mb file. Hopefully its Old world blues...