The protein levels on oats, barley, and wheat tend to be around 12%. that's why you need the alfalfa hay. (or fresh). The grains are there for various vitamins and minerals, but it's actually the alfalfa that should be the mainstay. BOSS will up your protein levels quite nicely...but remember it's the amount of inner seed and NOT the whole hull that you need to calculate. Inner seed (the meat) is 26%ish. With the hull, which is almost pure fiber, it drops down to 12%ish for the whole seed (by volume).
A rabbit could survive quite nicely on JUST alfalfa, although nutrients and salts would be missing.
Oh. If you're feeding a grains/alfalfa based diet, you need to add a mineral salt wheel or block. check labels on the salt blocks that you see in a feed store. Deer licks would be the closest to "good for rabbit". Cattle blocks often have strange things in them JUST for cattle.