Oat hay or oat straw? Oat straw is useless as a feed. We used to overseed our hayfields with oats and cut the hay just before the oats headed out. It was very high protein, but didn't cause any problems.
A veterinarian at the University of Illinois who told me that his father's show Shires wintered on just oat straw would disagree. It is very safe, and low
protein. For horses with low metabolism and for horses bedded in it that nibble on it, it is good fodder and makes a good supplement that doesn't make them hot.
I have used it as bedding and I can see the oats in it.