You could switch and use the stuff in the field if you let it mature some so it isn't all short early growth oat grass. The difference is that when oat hay is harvested, it has been growing a while and so isn't the very rich young early growth. It's the same for any early growth grass. The young early shoots are very rich. I just mentioned also giving hay as well because if you are only giving the rich young oat grass, then too much, like in the amounts of hay rabbits normally eat, would probably not be good. You want a mix of leafy grass(hay) and more mature hay with some stalks for the indigestible fiber. So if you can collect a variety of growth stages from the field so you aren't just feeding only the young early growth, then that should be fine to feed to your rabbit. You still want to introduce gradually if it is the young early growth, and you also want to make sure this oat grass hasn't been sprayed with chemicals at all.
I don't know if it's possible where you are, but usually you can find quality grass hay that would normally be used to feed horses. You want something that has NEVER been wet and gotten moldy, preferably no weeds, and not dusty. Good quality horse hay is usually fine for rabbits too.
If your bun isn't eating the hay then you do need to make sure it is getting enough nutrition. If the oat hay is bad at all, this may be why your rabbit won't eat it. So if you can get the fresh oat grass introduced and feed enough of that, then it should be fine. Or you will need to find other hay, or even gradually increase the pellet amount. But you don't want your rabbit to be on reduced pellets longterm, when it isn't eating any hay at all, as this isn't healthy for your rabbit and will start to lose weight. Hay/grass is preferable, but feeding more pellets is better than your bun not getting enough food.
