Barley I believe would be more conditioning than oats too 😊
Oats are high in fibre and well suited for the horses digestive system, as with anything they should be fed as a part of a balanced diet as alone oats are very low in calcium. As any feed they must also be fed appropriately for the amount of work your horse is in, otherwise you can cause problems like Asoturia, on top of that Oats can be quite heating.
I hope this helps, this is all I know about Oats 😊
He is currently on the thin side and I was told that oats will help fatten him up. Is that true? 😊
You do not have to cook, more vitamins will get while they are raw. They help to increase energy, they are a good source of protein.
I didn't read your question fully so here is the other half: Oats give more energy to the horse and gives them the energy to work (at least this is what I heard from the vet). It gives them a bit more "boost" than just hay. You shouldn't feed the horse too much oat but adding a bit to their hay is fine. I really don't know any disadvantages but as I said you shouldn't five the horse too many oats. I hope this helped!
Horses can eat oats raw. You don't need to cook it. This is what I know. I don't think it would be a problem if you cook it though