My circa 1979 Pony Club manual states that good quality oat straw may be used as whole or part of a horse's forage ration and that it is an ideal bulk forage for good-doers or in place of an otherwise poor hay crop.
That's what I thought - looks like that's what we'll be getting this year, as our landlord values it less than wheat straw so has more that we can have!
Makes lovely beds and doesn't matter too much if they eat it as long as it's nice and clean, not dusty. That and barley straw are my choice rather than wheat.