I've never tried the oat hay before. I've used Bermuda, brome, timothy, and orchard, and given some alfalfa as a treat hay. I will have to order some, I'm running low on the Bermuda/brome mix I give my buns.
One interesting thing, the younger any hay is cut, whether grass hay or legume, the higher the protein levels. As the plant matures, the protein is replaced by cellulose and starches. This can make a larger difference- even between an 8% protein level and a 14% protein level. So perhaps that is why there is so much different information- because some crude analyses will give different levels for the same kind of hay. For instance, I've seen a particular brand of grass hay give an analysis of 12% protein- just timothy hay, no legume hay mixed in. And yet there are some (most) that give an analysis of 6-8%.
I am still curious, however, and will read up some more on the subject when I have a little more time.
Edit: Because I am silly and misspelled something.