It's nice enough feed....for big horses. We fed some the one year when we had only the Morgans, no minis. The thing is, they go for the oat heads & leave the stem & leaves portion, so you have to be sure & give them only the exact amount that they need to eat, & then they have to eat the entire thing.
I was offered some this fall, but I said no, I didn't feel it was suitable for our Minis. They'd go for the oat heads, of course, but the stalks & leaves would just be too coarse for them.
It does pay, too, to get green feed (that's what "oat hay" is generally called here, though of course green feed can also be barley &/or millet) tested. Green feed is very common as cattle feed here; two years ago the cattle that were being fed a lot of green feed really lost weight--the feed just did not have any nutrients in it; it looked ideal, but really wasn't.