Llamas and oat hay
It is always hard to diagnose a problem from a distance. However, if the animals had substantial weight loss prior to death, one main concern would be the nutritional quality of the oat hay. Does the hay have a lot of oat heads present with oats present in in the heads? If not, it may have a nutritional profile similar to straw which can not provide adequate nutrition to an animal, especially in the winter. The best is to have you veterinarian examine the feed and send if off for nutritional testing and possibly mycotoxin testing if mold is a major concern. Also, if any more animals die, a necropsy should be done.